View source for Astronomy:Asteroid

{{Short description|Minor planets found within the inner Solar System}}
{{Multiple image
| perrow            = 2
| total_width       = 315
| image1            = Eros_-_PIA02923_(color).jpg
| alt1              = 433 Eros photographed by NEAR Shoemaker
| image2            = 243_Ida_-_August_1993_(16366655925).jpg
| alt2              = Galileo image of 243 Ida (the dot to the right is its moon Dactyl)
| image3            = Ceres - RC3 - Haulani Crater (22381131691) (cropped).jpg
| alt3              = Dawn image of the dwarf planet Ceres
| image4            = Bennu_mosaic_OSIRIS-REx_(square).png
| alt4              = OSIRIS-REx image of 101955 Bennu, a rubble-pile asteroid
| footer            = Images of visited asteroids illustrating their differences: (top row) [[Astronomy:433 Eros|433 Eros]] and [[Astronomy:243 Ida|243 Ida]] with its moon Dactyl, (bottom row) [[Astronomy:Ceres (dwarf planet)|Ceres]] and [[Astronomy:101955 Bennu|101955 Bennu]].
''Sizes are not to scale.''
}}

An '''asteroid''' is a [[Astronomy:Minor planet|minor planet]]—an object that is neither a [[Astronomy:Planet|true planet]] nor a [[Astronomy:Comet|comet]]—that orbits within the [[Astronomy:Solar System#Inner Solar System|inner Solar System]]. They are rocky, metallic, or icy bodies with no atmosphere. The size and shape of asteroids vary significantly, ranging from small rubble piles under a kilometer across to [[Astronomy:Ceres (dwarf planet)|Ceres]], a [[Astronomy:Dwarf planet|dwarf planet]] almost 1000&nbsp;km in diameter.

Of the roughly one million known asteroids,<ref>{{citation-attribution|1={{cite web |title=Asteroids |url=https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/overview/ |publisher=NASA Solar System Exploration |access-date=29 March 2022}} }}</ref> the greatest number are located between the orbits of [[Astronomy:Mars|Mars]] and [[Astronomy:Jupiter|Jupiter]], approximately 2 to 4 [[Astronomy:Astronomical unit|AU]] from the Sun, in a region known as the main [[Astronomy:Asteroid belt|asteroid belt]]. Asteroids are generally classified to be of three types: [[Astronomy:C-type asteroid|C-type]], [[Astronomy:M-type asteroid|M-type]], and [[Astronomy:S-type asteroid|S-type]]. These were named after and are generally identified with [[Chemistry:Carbon|carbon]]aceous, [[Chemistry:Metal|metal]]lic, and [[Chemistry:Silica|silica]]ceous compositions, respectively. The size of asteroids varies greatly; the largest, [[Astronomy:Ceres (dwarf planet)|Ceres]], is almost {{cvt|1000|sigfig=1|km}} across and qualifies as a [[Astronomy:Dwarf planet|dwarf planet]]. The total mass of all the asteroids combined is only 3% that of Earth's Moon. The majority of main belt asteroids follow slightly elliptical, stable orbits, revolving in the same direction as the Earth and taking from three to six years to complete a full circuit of the Sun.<ref name="press-kit">{{citation-attribution|1={{cite web |title=Asteroids (from the NEAR press kit) |url=https://nssdc.gsfc.nasa.gov/planetary/text/asteroids.txt |website=nssdc.gsfc.nasa.gov |access-date=29 March 2022}} }}</ref>

Asteroids have been historically observed from Earth; the ''[[Engineering:Galileo (spacecraft)|Galileo]]'' spacecraft provided the first close observation of an asteroid. Several dedicated missions to asteroids were subsequently launched by [[Organization:NASA|NASA]] and [[Organization:JAXA|JAXA]], with plans for other missions in progress. NASA's ''[[Astronomy:NEAR Shoemaker|NEAR Shoemaker]]'' studied [[Astronomy:433 Eros|Eros]], and ''[[Engineering:Dawn (spacecraft)|Dawn]]'' observed [[Astronomy:4 Vesta|Vesta]] and [[Astronomy:1 Ceres|Ceres]]. JAXA's missions ''[[Astronomy:Hayabusa|Hayabusa]]'' and ''[[Astronomy:Hayabusa2|Hayabusa2]]'' studied and returned samples of [[Astronomy:25143 Itokawa|Itokawa]] and [[Astronomy:162173 Ryugu|Ryugu]], respectively. [[Astronomy:OSIRIS-REx|OSIRIS-REx]] studied [[Astronomy:101955 Bennu|Bennu]], collecting a sample in 2020 which was delivered back to Earth in 2023. NASA's ''[[Engineering:Lucy (spacecraft)|Lucy]]'', launched in 2021, will study ten different asteroids, two from the [[Astronomy:Asteroid belt|main belt]] and eight [[Astronomy:Jupiter trojan|Jupiter trojan]]s. ''[[Engineering:Psyche (spacecraft)|Psyche]]'', launched in October 2023, will study a metallic [[Astronomy:16 Psyche|asteroid of the same name]].

Near-Earth asteroids can threaten all life on the planet; an asteroid [[Astronomy:Impact event|impact event]] resulted in the [[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction]]. Different asteroid deflection strategies have been proposed; the [[Astronomy:Double Asteroid Redirection Test|Double Asteroid Redirection Test]] spacecraft, or DART, was launched in 2021 and intentionally impacted [[Astronomy:Dimorphos|Dimorphos]] in September 2022, successfully altering its orbit by crashing into it.

== History of observations ==
Despite their large numbers, asteroids are a relatively recent discovery, with the first known asteroid—Ceres—only being identified in 1801.<ref name="cunningham2001"/> Only one asteroid, [[Astronomy:4 Vesta|4 Vesta]], which has a relatively [[Earth:Albedo|reflective surface]], is normally visible to the naked eye. When favorably positioned, 4 Vesta can be seen in dark skies. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short amount of time.<ref name=SPACE-2004-02-04/> {{As of|2022|4|df=dmy}}, the [[Organization:Minor Planet Center|Minor Planet Center]] had data on 1,199,224 minor planets in the inner and outer Solar System, of which about 614,690 had enough information to be given numbered designations.<ref name=MPCcount/>

===Discovery of Ceres===
In 1772, German astronomer Johann Elert Bode, citing [[Biography:Johann Daniel Titius|Johann Daniel Titius]], published a numerical procession known as the [[Astronomy:Titius–Bode law|Titius–Bode law]] (now discredited). Except for an unexplained gap between Mars and Jupiter, Bode's formula seemed to predict the orbits of the known planets.<ref name="hoskin" /><ref name="Hogg1948">{{cite journal |last=Hogg |first=Helen Sawyer |title=The Titius-Bode Law and the Discovery of Ceres |journal=Journal of the Royal Astronomical Society of Canada |volume=242 |pages=241–246 |year=1948 |bibcode=1948JRASC..42..241S |url=http://articles.adsabs.harvard.edu//full/1948JRASC..42..241S/0000244.000.html |access-date=18 July 2021 |archive-date=18 July 2021 |archive-url=https://web.archive.org/web/20210718191659/http://articles.adsabs.harvard.edu//full/1948JRASC..42..241S/0000244.000.html |url-status=live }}</ref> He wrote the following explanation for the existence of a "missing planet":

<blockquote>This latter point seems in particular to follow from the astonishing relation which the known six planets observe in their distances from the Sun. Let the distance from the Sun to Saturn be taken as 100, then Mercury is separated by 4 such parts from the Sun. Venus is 4 + 3 = 7. The Earth 4 + 6 = 10. Mars 4 + 12 = 16. Now comes a gap in this so orderly progression. After Mars there follows a space of 4 + 24 = 28 parts, in which no planet has yet been seen. Can one believe that the Founder of the universe had left this space empty? Certainly not. From here we come to the distance of Jupiter by 4 + 48 = 52 parts, and finally to that of Saturn by 4 + 96 = 100 parts.<ref name=discovery>{{cite book |last1=Foderà Serio |first1=G. |last2=Manara |first2=A. |last3=Sicoli |first3=P. |chapter=Giuseppe Piazzi and the Discovery of Ceres |chapter-url=https://www.lpi.usra.edu/books/AsteroidsIII/pdf/3027.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.lpi.usra.edu/books/AsteroidsIII/pdf/3027.pdf |archive-date=2022-10-09 |url-status=live |pages=17–24 |bibcode=2002aste.book...17F |editor1=W. F. Bottke Jr. |editor2=A. Cellino |editor3=P. Paolicchi |editor4=R. P. Binzel |title=Asteroids III |date=2002 |publisher=University of Arizona Press |location=Tucson |isbn=978-0-8165-4651-0}}</ref></blockquote>

Bode's formula predicted another planet would be found with an orbital radius near 2.8 [[Astronomy:Astronomical unit|astronomical unit]]s (AU), or 420&nbsp;million&nbsp;km, from the Sun.<ref name="Hogg1948" /> The Titius–Bode law got a boost with William Herschel's discovery of [[Astronomy:Uranus|Uranus]] near the predicted distance for a planet beyond [[Astronomy:Saturn|Saturn]].<ref name="hoskin" /> In 1800, a group headed by Franz Xaver von Zach, editor of the German astronomical journal ''Monatliche Correspondenz'' (Monthly Correspondence), sent requests to 24 experienced astronomers (whom he dubbed the "[[Astronomy:Celestial police|celestial police]]"),<ref name="Hogg1948" /> asking that they combine their efforts and begin a methodical search for the expected planet.<ref name="Hogg1948" /> Although they did not discover Ceres, they later found the asteroids [[Astronomy:2 Pallas|2 Pallas]], [[Astronomy:3 Juno|3 Juno]] and [[Astronomy:4 Vesta|4 Vesta]].<ref name="Hogg1948" />

One of the astronomers selected for the search was [[Biography:Giuseppe Piazzi|Giuseppe Piazzi]], a Catholic [[Religion:Priest|priest]] at the Academy of Palermo, Sicily. Before receiving his invitation to join the group, Piazzi discovered Ceres on 1 January 1801.<ref name="NASA-20160126">{{cite web |last=Landau |first=Elizabeth |title=Ceres: Keeping Well-Guarded Secrets for 215 Years |url=http://www.jpl.nasa.gov/news/news.php?feature=4824 |date=26 January 2016 |work=NASA |access-date=26 January 2016 |archive-date=24 May 2019 |archive-url=https://web.archive.org/web/20190524043553/https://www.jpl.nasa.gov/news/news.php?feature=4824 |url-status=live }}</ref> He was searching for "the 87th [star] of the Catalogue of the Zodiacal stars of Mr la Caille",<ref name="hoskin"/> but found that "it was preceded by another".<ref name="hoskin">{{cite web|last=Hoskin |first=Michael |date=26 June 1992 |url=http://www.astropa.unipa.it/HISTORY/hoskin.html |title=Bode's Law and the Discovery of Ceres |publisher=Observatorio Astronomico di Palermo "Giuseppe S. Vaiana" |access-date=5 July 2007 |archive-url=https://web.archive.org/web/20071116022100/http://www.astropa.unipa.it/HISTORY/hoskin.html |archive-date=16 November 2007 |url-status=live }}</ref> Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet:<ref name="Forbes1971">{{cite journal |last=Forbes |first=Eric G. |title=Gauss and the Discovery of Ceres |journal=Journal for the History of Astronomy |volume=2 |issue=3 |pages=195–199 |year=1971 |bibcode=1971JHA.....2..195F |doi=10.1177/002182867100200305 |s2cid=125888612 |url=http://adsabs.harvard.edu/full/1971JHA.....2..195F |access-date=18 July 2021 |archive-date=18 July 2021 |archive-url=https://web.archive.org/web/20210718200510/http://adsabs.harvard.edu/full/1971JHA.....2..195F |url-status=live }}</ref>

<blockquote>The light was a little faint, and of the colour of [[Astronomy:Jupiter|Jupiter]], but similar to many others which generally are reckoned of the eighth [[Astronomy:Apparent magnitude|magnitude]]. Therefore I had no doubt of its being any other than a fixed star. [...] The evening of the third, my suspicion was converted into certainty, being assured it was not a fixed star. Nevertheless before I made it known, I waited till the evening of the fourth, when I had the satisfaction to see it had moved at the same rate as on the preceding days.<ref name="hoskin"/></blockquote>

Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his work. He announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode in Berlin.<ref name="cunningham2001">{{cite book |first=Clifford J. |last=Cunningham |title=The first asteroid: Ceres, 1801–2001 |url=https://books.google.com/books?id=CXdMPwAACAAJ |year=2001 |publisher=Star Lab Press |isbn=978-0-9708162-1-4 |access-date=23 October 2015 |archive-date=29 May 2016 |archive-url=https://web.archive.org/web/20160529144326/https://books.google.com/books?id=CXdMPwAACAAJ |url-status=live }}</ref> He reported it as a comet but "since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet".<ref name="hoskin" /> In April, Piazzi sent his complete observations to Oriani, Bode, and French astronomer Jérôme Lalande. The information was published in the September 1801 issue of the ''Monatliche Correspondenz''.<ref name="Forbes1971" />

By this time, the apparent position of Ceres had changed (mostly due to Earth's motion around the Sun), and was too close to the Sun's glare for other astronomers to confirm Piazzi's observations. Toward the end of the year, Ceres should have been visible again, but after such a long time it was difficult to predict its exact position. To recover Ceres, mathematician [[Biography:Carl Friedrich Gauss|Carl Friedrich Gauss]], then 24 years old, developed an [[Astronomy:Gauss's method|efficient method]] of [[Astronomy:Orbit determination|orbit determination]].<ref name="Forbes1971" /> In a few weeks, he predicted the path of Ceres and sent his results to von Zach. On 31 December 1801, von Zach and fellow celestial policeman Heinrich W. M. Olbers found Ceres near the predicted position and thus recovered it.<ref name="Forbes1971" /> At 2.8 AU from the Sun, Ceres appeared to fit the Titius–Bode law almost perfectly; however, Neptune, once discovered in 1846, was 8 AU closer than predicted, leading most astronomers to conclude that the law was a coincidence.<ref>{{cite book|title=The Titius-Bode Law of Planetary Distances: Its History and Theory|url=https://books.google.com/books?id=NneoBQAAQBAJ&q=bode+law+neptune+coincidence+1846&pg=PP1|publisher=Pergamon Press|year=1972|author=Michael Martin Nieto|isbn = 978-1-4831-5936-2|access-date=23 September 2021|archive-date=29 September 2021|archive-url=https://web.archive.org/web/20210929081229/https://books.google.co.uk/books?hl=en&lr=&id=NneoBQAAQBAJ&oi=fnd&pg=PP1&dq=bode+law+neptune+coincidence+1846&ots=LIplNAOXco&sig=qAF2y5xXTivecmSP_fjGCDA9Sx4&redir_esc=y|url-status=live}}</ref> Piazzi named the newly discovered object ''Ceres Ferdinandea,'' "in honor of the patron goddess of Sicily and of King Ferdinand of Bourbon".<ref name=discovery/>

===Further search===
[[File:Moon and Asteroids 1 to 10.svg|thumb|left|Sizes of the first ten discovered asteroids, compared to the Moon]]

Three other asteroids ([[Astronomy:2 Pallas|2 Pallas]], [[Astronomy:3 Juno|3 Juno]], and [[Astronomy:4 Vesta|4 Vesta]]) were discovered by von Zach's group over the next few years, with Vesta found in 1807.<ref name="Hogg1948" /> No new asteroids were discovered until 1845. Amateur astronomer [[Biography:Karl Ludwig Hencke|Karl Ludwig Hencke]] started his searches of new asteroids in 1830, and fifteen years later, while looking for Vesta, he found the asteroid later named [[Astronomy:5 Astraea|5 Astraea]]. It was the first new asteroid discovery in 38 years. [[Biography:Carl Friedrich Gauss|Carl Friedrich Gauss]] was given the honor of naming the asteroid. After this, other astronomers joined; 15 asteroids were found by the end of 1851. In 1868, when [[Biography:James Craig Watson|James Craig Watson]] discovered the 100th asteroid, the French Academy of Sciences engraved the faces of Karl Theodor Robert Luther, [[Biography:John Russell Hind|John Russell Hind]], and Hermann Goldschmidt, the three most successful asteroid-hunters at that time, on a commemorative medallion marking the event.<ref name="dawn-community">{{citation-attribution|1={{cite web |title=Dawn Community |url=http://dawn.jpl.nasa.gov/DawnCommunity/flashbacks/fb_09.asp |website=jpl.nasa.gov |publisher=JPL NASA |access-date=8 April 2022 |date=21 May 2009|archive-url=https://web.archive.org/web/20090521235728/http://dawn.jpl.nasa.gov/DawnCommunity/flashbacks/fb_09.asp |archive-date=21 May 2009 }} }}</ref>

In 1891, Max Wolf pioneered the use of [[Astrophotography|astrophotography]] to detect asteroids, which appeared as short streaks on long-exposure photographic plates.<ref name="dawn-community"/> This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248&nbsp;asteroids, beginning with [[Astronomy:323 Brucia|323 Brucia]],<ref>{{cite web |title=Dawn Classrooms – Biographies |url=http://dawn.jpl.nasa.gov/DawnClassrooms/1_hist_dawn/bio.asp#wolf |website=dawn.jpl.nasa.gov |publisher=JPL NASA |access-date=8 April 2022 |date=18 June 2009 |archive-url=https://web.archive.org/web/20090618143655/http://dawn.jpl.nasa.gov/DawnClassrooms/1_hist_dawn/bio.asp#wolf |archive-date=18 June 2009 |url-status=dead}}</ref> whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them, some calling them "vermin of the skies",<ref>{{cite web |last=Friedman |first=Lou |title=Vermin of the Sky |website=The Planetary Society |url=http://www.planetary.org/blogs/guest-blogs/lou-friedman/20130219-vermin-of-the-sky.html}}</ref> a phrase variously attributed to Eduard Suess<ref>{{cite magazine |last=Hale |first=George E. |series=Address at the semi-centennial of the Dearborn Observatory |title=Some Reflections on the Progress of Astrophysics |magazine=Popular Astronomy |date=1916 |volume=24 |pages=550–558 [555] |bibcode= 1916PA.....24..550H |bibcode-access=free}}</ref> and Edmund Weiss.<ref>{{cite journal |last=Seares |first=Frederick H. |title=Address of the Retiring President of the Society in Awarding the Bruce Medal to Professor Max Wolf |journal=Publications of the Astronomical Society of the Pacific |year=1930 |volume=42 |issue=245 | pages=5–22 [10] |bibcode=1930PASP...42....5S |bibcode-access=free |doi=10.1086/123986 |doi-access=free}}</ref> Even a century later, only a few thousand asteroids were identified, numbered and named.

=== 19th and 20th centuries ===
[[File:Known NEAs.png|thumb|Cumulative discoveries of just the near-Earth asteroids known by size, 1980–2022]]

In the past, asteroids were discovered by a four-step process. First, a region of the sky was [[Astrophotography|photographed]] by a wide-field [[Astronomy:Telescope|telescope]] or [[Astronomy:Astrograph|astrograph]]. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or [[Physics:Photographic plate|plates]] of the same region were viewed under a [[Stereoscope|stereoscope]]. A body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.<ref>{{cite web |last=Chapman |first=Mary G. |date=17 May 1992 |title=Carolyn Shoemaker, planetary astronomer and most successful 'comet hunter' to date |publisher=USGS |department=Astrogeology |url=https://astrogeology.usgs.gov/About/People/CarolynShoemaker |access-date=15 April 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080302124131/http://astrogeology.usgs.gov/About/People/CarolynShoemaker/ |archive-date=2008-03-02}}</ref>

These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a [[Astronomy:Provisional designation in astronomy|provisional designation]], made up of the year of discovery, a letter representing the half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: {{mp|1998 FJ|74}}). The last step is sending the locations and time of observations to the [[Organization:Minor Planet Center|Minor Planet Center]], where computer programs determine whether an apparition ties together earlier apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the [[Organization:International Astronomical Union|International Astronomical Union]].<ref>{{cite web |title=ESA Science & Technology – Asteroid numbers and names |url=https://sci.esa.int/web/home/-/30244-asteroid-numbers-and-names |website=sci.esa.int |access-date=13 April 2022}}</ref>

== Naming ==
[[File:Asteroid20130318-full.jpg|thumb|right|[[Astronomy:2013 EC|2013 EC]], shown here in radar images, has a provisional designation]]

By 1851, the Royal Astronomical Society decided that asteroids were being discovered at such a rapid rate that a different system was needed to categorize or name asteroids. In 1852, when de Gasparis discovered the twentieth asteroid, Benjamin Valz gave it a name and a number designating its rank among asteroid discoveries, [[Astronomy:20 Massalia|20 Massalia]]. Sometimes asteroids were discovered and not seen again. So, starting in 1892, new asteroids were listed by the year and a capital letter indicating the order in which the asteroid's orbit was calculated and registered within that specific year. For example, the first two asteroids discovered in 1892 were labeled 1892A and 1892B. However, there were not enough letters in the alphabet for all of the asteroids discovered in 1893, so 1893Z was followed by 1893AA. A number of variations of these methods were tried, including designations that included year plus a Greek letter in 1914. A simple chronological numbering system was established in 1925.<ref name="dawn-community"/><ref>{{cite web |title=New- And Old-Style Minor Planet Designations |url=http://www.cfa.harvard.edu/iau/info/OldDesDoc.html |website=cfa.harvard.edu |publisher=Harvard |access-date=8 April 2022 |date=22 August 2009 |archive-url=https://web.archive.org/web/20090822195033/http://www.cfa.harvard.edu/iau/info/OldDesDoc.html |archive-date=22 August 2009 |url-status=dead}}</ref>

Currently all newly discovered asteroids receive a [[Astronomy:Provisional designation in astronomy|provisional designation]] (such as {{mpl|2002 AT|4}}) consisting of the year of discovery and an alphanumeric code indicating the [[Physics:Half-month|half-month]] of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. {{nowrap|[[Astronomy:433 Eros|433 Eros]]}}). The formal naming convention uses parentheses around the number—e.g. (433)&nbsp;Eros—but dropping the parentheses is quite common. Informally, it is also common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text.<ref name=OpenUNamingAstrds/> In addition, names can be proposed by the asteroid's discoverer, within guidelines established by the International Astronomical Union.<ref name=PlanSocNameGuides/>

=== Symbols ===
{{Main|Astronomy:Astronomical symbols}}
The first asteroids to be discovered were assigned iconic symbols like the ones traditionally used to designate the planets. By 1855 there were two dozen asteroid symbols, which often occurred in multiple variants.<ref name=Gould-1852/>

In 1851, after the fifteenth asteroid, [[Astronomy:15 Eunomia|Eunomia]], had been discovered, Johann Franz Encke made a major change in the upcoming 1854 edition of the ''[[Astronomy:Berliner Astronomisches Jahrbuch|Berliner Astronomisches Jahrbuch]]'' (BAJ, ''Berlin Astronomical Yearbook''). He introduced a disk (circle), a traditional symbol for a star, as the generic symbol for an asteroid. The circle was then numbered in order of discovery to indicate a specific asteroid. The numbered-circle convention was quickly adopted by astronomers, and the next asteroid to be discovered ([[Astronomy:16 Psyche|16 Psyche]], in 1852) was the first to be designated in that way at the time of its discovery. However, Psyche was given an iconic symbol as well, as were a few other asteroids discovered over the next few years. [[Astronomy:20 Massalia|20 Massalia]] was the first asteroid that was not assigned an iconic symbol, and no iconic symbols were created after the 1855 discovery of [[Astronomy:37 Fides|37 Fides]].{{efn|Except for Pluto and, in the astrological community, for a few outer bodies such as [[Astronomy:2060 Chiron|2060 Chiron]].}}<ref name=Hilton-2011-a/>

== Terminology{{anchor|Terminology}}<!-- Linked from "Comet" --> ==
{{Multiple image
| direction         = vertical
| image1            = Asteroidsscale.jpg
| caption1          = A composite image, to the same scale, of the asteroids imaged at high resolution prior to 2012. They are, from largest to smallest: [[Astronomy:4 Vesta|4 Vesta]], [[Astronomy:21 Lutetia|21 Lutetia]], [[Astronomy:253 Mathilde|253 Mathilde]], [[Astronomy:243 Ida|243 Ida]] and its moon Dactyl, [[Astronomy:433 Eros|433 Eros]], [[Astronomy:951 Gaspra|951 Gaspra]], [[Astronomy:2867 Šteins|2867 Šteins]], [[Astronomy:25143 Itokawa|25143 Itokawa]].
| image2            = Ceres and Vesta, Moon size comparison.jpg
| caption2          = [[Astronomy:4 Vesta|Vesta]] (left), with [[Astronomy:Ceres (dwarf planet)|Ceres]] (center) and the [[Astronomy:Moon|Moon]] (right) shown to scale
| align             = right
}}

The first discovered asteroid, [[Astronomy:Ceres (dwarf planet)|Ceres]], was originally considered a new planet.{{efn|Ceres is the largest asteroid and now classified as a [[Astronomy:Dwarf planet|dwarf planet]]. All other asteroids are now classified as [[Astronomy:Small Solar System body|small Solar System bodies]] along with comets, centaurs, and the smaller trans-Neptunian objects.}} It was followed by the discovery of other similar bodies, which with the equipment of the time appeared to be points of light like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term ''asteroid'',{{efn|In an oral presentation,<ref>{{cite conference |title=HADII Abstracts |conference=HAD Meeting with DPS |place=Denver, CO |date=October 2013 |url=http://had.aas.org/meetings/2013bAbstracts.html#HADII |url-status=dead |access-date=14 October 2013 |archive-url=https://web.archive.org/web/20140901143955/http://had.aas.org/meetings/2013bAbstracts.html#HADII |archive-date=1 September 2014}}</ref> Clifford Cunningham presented his finding that the word was coined by Charles Burney, Jr., the son of a friend of Herschel.<ref>{{cite news |first=Robert |last=Nolin |date=8 October 2013 |title=Local expert reveals who really coined the word 'asteroid' |newspaper=Sun-Sentinel |url=http://www.sun-sentinel.com/news/broward/fl-asteroid-word-origin-20131008,0,501498,full.story |archive-url=https://archive.today/20141130155012/http://www.sun-sentinel.com/news/broward/fl-asteroid-word-origin-20131008,0,501498,full.story |url-status=dead |archive-date=30 November 2014 |access-date=10 October 2013 }}</ref><ref>{{cite web |title=Who really invented the word 'Asteroid' for space rocks? |last=Wall |first=Mike |website=Space.com |date=10 January 2011 |url=http://www.space.com/10593-post-william-herschel-coin-term-asteroid.html |access-date=10 October 2013}}</ref>}} coined in Greek as ἀστεροειδής, or ''asteroeidēs'', meaning 'star-like, star-shaped', and derived from the Ancient Greek {{lang|grc|ἀστήρ}} ''astēr'' 'star, planet'. In the early second half of the 19th century, the terms ''asteroid'' and ''planet'' (not always qualified as "minor") were still used interchangeably.{{efn|For example, the ''Annual of Scientific Discovery'': "Professor J. Watson has been awarded by the Paris Academy of Sciences, the astronomical prize, Lalande foundation, for the discovery of eight new asteroids in one year. The planet [[Astronomy:110 Lydia|Lydia]] (No. 110), discovered by M. Borelly at the Marseilles Observatory [...] M. Borelly had previously discovered two planets bearing the numbers 91 and 99 in the system of asteroids revolving between Mars and Jupiter".<ref>{{cite book |url=https://books.google.com/books?id=NAMAAAAAMAAJ&pg=PA316 |via=Google Books |title=Annual of Scientific Discovery |year=1871 |page=316}}</ref><br />The ''Universal English Dictionary'' (John Craig, 1869) lists the asteroids (and gives their pronunciations) up to [[Astronomy:64 Angelina|64 Angelina]], along with the definition "one of the recently-discovered planets." At this time it was common to anglicize the spellings of the names, e.g. "Aglaia" for [[Astronomy:47 Aglaja|47 Aglaja]] and "Atalanta" for [[Astronomy:36 Atalante|36 Atalante]].}}

Traditionally, small bodies orbiting the Sun were classified as [[Astronomy:Comet|comet]]s, asteroids, or [[Astronomy:Meteoroid|meteoroid]]s, with anything smaller than one meter across being called a meteoroid. The term ''asteroid'' never had a formal definition,<ref>{{cite book |editor1-last=Bottke |editor1-first=William F. |editor1-link=William F. Bottke |editor2-last=Cellino |editor2-first=Alberto |editor3-last=Paolicchi |editor3-first=Paolo |editor4-last=Binzel |editor4-first=Richard P. |editor4-link=Richard P. Binzel |title=Asteroids III |date=2002 |publisher=University of Arizona Press |location=Tucson |isbn=978-0-8165-4651-0 |url=https://books.google.com/books?id=JwHTyO6IHh8C&pg=PA670 |access-date=30 March 2022| page=[https://books.google.com/books?id=JwHTyO6IHh8C&pg=PA670 670] |quote=Since no formal definitions of comets and asteroids exist...}}</ref> with the broader term ''[[Astronomy:Small Solar System bodies|small Solar System bodies]]'' being preferred by the [[Organization:International Astronomical Union|International Astronomical Union]] (IAU).<ref>{{cite web |title=Resolution B5 Definition of a Planet in the Solar System |url=https://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf |archive-date=2022-10-09 |url-status=live |publisher=The Minor Planet Center |access-date=30 April 2022 |quote=All other objects (These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.), except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".}}</ref> As no IAU definition exists, ''asteroid'' can be defined as "an irregularly shaped rocky body orbiting the Sun that does not qualify as a planet or a dwarf planet under the IAU definitions of those terms".<ref>{{cite book |last1=Harris |first1=Alan W. |title=Encyclopedia of Astrobiology |chapter=Asteroid |date=2011 |pages=102–112 |doi=10.1007/978-3-642-11274-4_116|isbn=978-3-642-11271-3 }}</ref>

When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until ''small Solar System body'' was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to [[Physics:Outgassing|sublimation]] of near-surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface [[Earth:Volatile (astrogeology)|volatile ices]] and become asteroid-like. A further distinction is that comets typically have more eccentric orbits than most asteroids; "asteroids" with notably eccentric orbits are probably dormant or extinct comets.<ref>{{cite web |last1=Weissman |first1=Paul R. |last2=Bottke |first2=William F. Jr. |last3=Levinson |first3=Harold F. |title=Evolution of Comets into Asteroids |publisher=Southwest Research Institute |department=Planetary Science Directorate |date=2002 |url=http://www.boulder.swri.edu/~hal/PDF/asteroids3.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.boulder.swri.edu/~hal/PDF/asteroids3.pdf |archive-date=2022-10-09 |url-status=live |access-date= 3 August 2010}}</ref>

For almost two centuries, from the discovery of [[Astronomy:Ceres (dwarf planet)|Ceres]] in 1801 until the discovery of the first [[Astronomy:Centaur (minor planet)|centaur]], [[Astronomy:2060 Chiron|2060 Chiron]] in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as [[Astronomy:944 Hidalgo|944 Hidalgo]] ventured far beyond Jupiter for part of their orbit. When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called [[Astronomy:Centaur (minor planet)|centaurs]], they numbered them among the traditional asteroids. There was debate over whether these objects should be considered asteroids or given a new classification. Then, when the first [[Astronomy:Trans-Neptunian object|trans-Neptunian object]] (other than [[Astronomy:Pluto|Pluto]]), [[Astronomy:15760 Albion|15760 Albion]], was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: [[Astronomy:Kuiper belt|Kuiper-belt object]], [[Astronomy:Trans-Neptunian object|trans-Neptunian object]], scattered-disc object, and so on. They inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids.

The innermost of these are the [[Astronomy:Kuiper belt|Kuiper-belt objects]], called "objects" partly to avoid the need to classify them as asteroids or comets.<ref name=KBOasteroids>{{cite web |title=Are Kuiper Belt objects asteroids? |website=Ask an astronomer |publisher=Cornell University |url=http://curious.astro.cornell.edu/question.php?number=601 |url-status=dead |archive-url=https://web.archive.org/web/20090103110110/http://curious.astro.cornell.edu/question.php?number=601 |archive-date=3 January 2009}}</ref> They are thought to be predominantly comet-like in composition, though some may be more akin to asteroids.<ref>{{cite web | first=Nicholas M. Sr. |last=Short |title=Asteroids and Comets |publisher=NASA |department=Goddard Space Flight Center |url=http://rst.gsfc.nasa.gov/Sect19/Sect19_22.html |url-status=dead |archive-url=https://web.archive.org/web/20080925014037/http://rst.gsfc.nasa.gov///Sect19/Sect19_22.html |archive-date=25 September 2008}}</ref> Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional [[Astronomy:Comet nucleus|comet nuclei]]. (The much more distant [[Astronomy:Oort cloud|Oort cloud]] is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the [[Engineering:Stardust (spacecraft)|''Stardust'']] probe, are increasingly blurring the distinction between comets and asteroids,<ref>{{cite AV media |title=Comet dust seems more 'asteroidy' |medium=audio podcast |magazine=Scientific American |date=25 January 2008 |url=http://www.sciam.com/podcast/episode.cfm?id=ADD0878B-D6C3-3B70-7B5BC373545BB82D}}</ref> suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.<ref>{{cite magazine |title=Comet samples are surprisingly asteroid-like |magazine=New Scientist |date=24 January 2008 |url=https://www.newscientist.com/channel/solar-system/comets-asteroids/dn13224-comet-samples-are-surprisingly-asteroidlike.html}}</ref>

The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations.{{efn|For instance, a joint [[Organization:NASA|NASA]]–[[Organization:JPL|JPL]] public-outreach website states: {{blockquote|We include Trojans (bodies captured in Jupiter's 4th and 5th Lagrange points), Centaurs (bodies in orbit between Jupiter and Neptune), and trans-Neptunian objects (orbiting beyond Neptune) in our definition of "asteroid" as used on this site, even though they may more correctly be called "minor planets" instead of asteroids.<ref>{{Cite web |title=Asteroids |url=https://ssd.jpl.nasa.gov/?asteroids |archive-url=https://web.archive.org/web/20060614184348/https://ssd.jpl.nasa.gov/?asteroids |archive-date=14 June 2006 |department=Solar System Dynamics |publisher=[[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] |access-date=8 December 2021}}</ref>}}}} However, it is becoming increasingly common for the term ''asteroid'' to be restricted to minor planets of the inner Solar System.<ref name=KBOasteroids/> Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the [[Astronomy:Asteroid belt|asteroid belt]], [[Astronomy:Jupiter trojan|Jupiter trojan]]s, and [[Astronomy:Near-Earth object|near-Earth object]]s.

When the IAU introduced the class [[Astronomy:Small Solar System body|small Solar System bodies]] in 2006 to include most objects previously classified as minor planets and comets, they created the class of [[Astronomy:Dwarf planet|dwarf planet]]s for the largest minor planets—those that have enough mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally, the term 'Small Solar System Body' will be preferred."<ref>{{cite web |url=http://www.iau.org/public/themes/pluto/ |title=Pluto |series=Questions and Answers on Planets |publisher=International Astrophysical Union}}</ref> Currently only the largest object in the asteroid belt, [[Astronomy:Ceres (dwarf planet)|Ceres]], at about {{cvt|975|km|0}} across, has been placed in the dwarf planet category.<ref name=dwarf1>{{Cite web |title=Pluto and the Developing Landscape of Our Solar System |url=https://www.iau.org/public/themes/pluto/ |access-date=2022-04-13 |website=[[Organization:International Astronomical Union|International Astronomical Union]]}}</ref><ref name=dwarf2>{{Cite web |date=26 June 2019 |title=Exploration: Ceres |url=https://solarsystem.nasa.gov/planets/dwarf-planets/ceres/exploration |access-date=12 April 2022 |website=NASA Science: Solar System Exploration}}</ref>

== Formation ==
Many asteroids are the shattered remnants of [[Astronomy:Planetesimal|planetesimal]]s, bodies within the young Sun's solar nebula that never grew large enough to become [[Astronomy:Planet|planet]]s.<ref name=CNEOS-FAQ/> It is thought that planetesimals in the asteroid belt evolved much like the rest of objects in the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately {{cvt|120|km|0}} in diameter [[Physics:Accretion (astrophysics)|accreted]] during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption.<ref>{{cite journal |last1=Bottke | first1=William F. Jr. |last2=Durda |first2=Daniel D. |last3=Nesvorny |first3=David |last4=Jedicke |first4=Robert |last5=Morbidelli |first5=Alessandro |last6=Vokrouhlicky |first6=David |last7=Levison |first7=Hal |year=2005 |title=The fossilized size distribution of the main asteroid belt |journal=Icarus |volume=175 |issue=1 |page=111 |doi=10.1016/j.icarus.2004.10.026 |bibcode=2005Icar..175..111B |url=http://astro.mff.cuni.cz/davok/papers/fossil05.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://astro.mff.cuni.cz/davok/papers/fossil05.pdf |archive-date=2022-10-09 |url-status=live}}</ref> Ceres and Vesta grew large enough to melt and [[Astronomy:Planetary differentiation|differentiate]], with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.<ref name=ACM>{{cite book |title=Asteroids, Comets, and Meteors |last=Kerrod |first=Robin |year=2000 |publisher=Lerner Publications Co. |isbn=978-0-585-31763-2 |url-access=registration |url=https://archive.org/details/asteroidscometsm00robi}}</ref>

In the [[Astronomy:Nice model|Nice model]], many [[Astronomy:Kuiper belt|Kuiper-belt objects]] are captured in the outer asteroid belt, at distances greater than 2.6&nbsp;AU. Most were later ejected by Jupiter, but those that remained may be the [[Astronomy:D-type asteroid|D-type asteroid]]s, and possibly include Ceres.<ref>{{cite journal |last1=McKinnon |first1=William |first2=B. |last2=McKinnon |year=2008 |title=On The Possibility of Large KBOs Being Injected into The Outer Asteroid Belt |journal=Bulletin of the American Astronomical Society |volume=40 |page=464 |bibcode=2008DPS....40.3803M}}</ref>

== Distribution within the Solar System ==
[[File:Inner solar system objects top view for wiki.png|thumb|A top view of asteroid group location in the inner solar system|350x350px]]
[[File:Inner solar system linear map.png|thumb|A map of planets and asteroid groups of the inner solar system. Distances from sun are to scale, object sizes are not.]]

Various dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and by the [[Yarkovsky effect]]. Significant populations include:

=== Asteroid belt ===
{{Main|Astronomy:Asteroid belt}}
The majority of known asteroids orbit within the asteroid belt between the orbits of [[Astronomy:Mars|Mars]] and [[Astronomy:Jupiter|Jupiter]], generally in relatively low-[[Astronomy:Orbital eccentricity|eccentricity]] (i.e. not very elongated) orbits. This belt is estimated to contain between 1.1 and 1.9&nbsp;million asteroids larger than {{cvt|1|km|1}} in diameter,<ref>{{cite press release
 | first1=Edward
 | last1=Tedesco
 | last2=Metcalfe
 | first2=Leo
 | date=4 April 2002
 | title=New study reveals twice as many asteroids as previously believed
 | publisher=European Space Agency
 | url=http://www.spaceref.com/news/viewpr.html?pid=7925
 | access-date=21 February 2008
 | archive-date=6 March 2023
 | archive-url=https://web.archive.org/web/20230306222828/https://spaceref.com/press-release/new-study-reveals-twice-as-many-asteroids-as-previously-believed/
 | url-status=dead
 }}</ref> and millions of smaller ones. These asteroids may be remnants of the [[Astronomy:Protoplanetary disk|protoplanetary disk]], and in this region the [[Physics:Accretion (astrophysics)|accretion]] of [[Astronomy:Planetesimal|planetesimal]]s into planets during the formative period of the Solar System was prevented by large gravitational perturbations by [[Astronomy:Jupiter|Jupiter]].

Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that reaching an asteroid without aiming carefully would be improbable. Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff. Over 200 asteroids are known to be larger than 100&nbsp;km,<ref>{{cite web
| last = Yeomans
| first = Donald K.
| date = April 26, 2007
| url = http://ssd.jpl.nasa.gov/sbdb_query.cgi
| title = JPL Small-Body Database Search Engine
| publisher = NASA JPL
| access-date = 2007-04-26
| at = Search for asteroids in the main belt regions with a diameter&nbsp;>100}}</ref> and a survey in the infrared wavelengths has shown that the asteroid belt has between 700,000 and 1.7&nbsp;million asteroids with a diameter of 1&nbsp;km or more.<ref>{{cite journal
|last1=Tedesco |first1= E. F.|last2=Desert |first2= F.-X.|name-list-style=amp| title=The Infrared Space Observatory Deep Asteroid Search
| journal=The Astronomical Journal
| date=2002
| volume=123
| issue=4
| pages=2070–2082
| bibcode=2002AJ....123.2070T| doi = 10.1086/339482
| doi-access=free
}}</ref> The [[Astronomy:Absolute magnitude|absolute magnitude]]s of most of the known asteroids are between 11 and 19, with the median at about 16.<ref name="mpc">{{cite web
| last = Williams
| first = Gareth
|date=September 25, 2010
| url = http://www.minorplanetcenter.org/iau/lists/MPDistribution.html
| title = Distribution of the Minor Planets
| publisher = Minor Planet Center
| access-date = 2010-10-27
}}</ref>

The total mass of the asteroid belt is estimated to be {{val|2.39e21}} kg, which is just 3% of the mass of the Moon; the mass of the Kuiper Belt and Scattered Disk is over 100 times as large.<ref name="Pitjeva2018">{{cite journal|last=Pitjeva|first=E. V.|title=Masses of the Main Asteroid Belt and the Kuiper Belt from the Motions of Planets and Spacecraft|journal=Solar System Research|volume=44|issue=8–9|pages=554–566|date=2018|arxiv=1811.05191|doi=10.1134/S1063773718090050|bibcode=2018AstL...44..554P|s2cid=119404378}}</ref> The four largest objects, Ceres, Vesta, Pallas, and Hygiea, account for maybe 62% of the belt's total mass, with 39% accounted for by Ceres alone.

=== Trojans ===
{{Main|Astronomy:Trojan (celestial body)}}
Trojans are populations that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two [[Astronomy:Lagrangian point|Lagrangian point]]s of stability, {{L4|nolink=yes}} and {{L5|nolink=yes}}, which lie 60° ahead of and behind the larger body.

In the Solar System, most known trojans share the [[Astronomy:Jupiter trojan|orbit of Jupiter]]. They are divided into the Greek camp at {{L4|nolink=yes}} (ahead of Jupiter) and the Trojan camp at {{L5|nolink=yes}} (trailing Jupiter). More than a million Jupiter trojans larger than one kilometer are thought to exist,<ref name=Yoshida2006>{{cite journal
|last1=Yoshida |first1=F.
|last2=Nakamura |first2=T.
|title=Size Distribution of Faint Jovian L4 Trojan Asteroids
|doi=10.1086/497571
|journal=The Astronomical Journal
|volume=130 |issue=6 |pages=2900–2911
|date=Dec 2005
|bibcode=2005AJ....130.2900Y
|doi-access=free
}}</ref> of which more than 7,000 are currently catalogued. In other planetary orbits only nine [[Astronomy:Mars trojan|Mars trojan]]s, 28 [[Astronomy:Neptune trojan|Neptune trojan]]s, two [[Astronomy:Uranus trojan|Uranus trojan]]s, and two [[Astronomy:Earth trojan|Earth trojan]]s, have been found to date. A temporary [[Astronomy:2013 ND15|Venus trojan]] is also known. Numerical orbital dynamics stability simulations indicate that Saturn and Uranus probably do not have any primordial trojans.<ref name=sheppard2006>{{cite journal
|last1=Sheppard |first1=Scott S.
|last2=Trujillo |first2=Chadwick A.
|title=A Thick Cloud of Neptune Trojans and their Colors
|doi=10.1126/science.1127173
|journal=Science
|volume=313 |issue=5786 |pages=511–514
|date=June 2006
|url=http://www.dtm.ciw.edu/users/sheppard/pub/Sheppard06NepTroj.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.dtm.ciw.edu/users/sheppard/pub/Sheppard06NepTroj.pdf |archive-date=2022-10-09 |url-status=live
|pmid=16778021 |bibcode= 2006Sci...313..511S
|s2cid=35721399
}}</ref>

=== Near-Earth asteroids ===

Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as ''Earth-crossers''. {{As of|2022|04}}, a total of 28,772&nbsp;near-Earth asteroids were known; 878 have a diameter of one kilometer or larger.<ref name=nasa_neo>{{cite web |title=Discovery Statistics |url=https://cneos.jpl.nasa.gov/stats/totals.html |website=CNEOS |access-date=14 April 2022}}</ref>

A small number of NEAs are extinct comets that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with [[Astronomy:Jupiter|Jupiter]].<ref name = "MorbidelliAstIII" /><ref>{{cite journal |title=What the physical properties of near-Earth asteroids tell us about sources of their origin? |author=D.F. Lupishko |author2=M. di Martino |author3=T.A. Lupishko |name-list-style=amp |journal=Kinematika I Fizika Nebesnykh Tel Supplimen |volume=3 |issue=3 |pages=213–216 |date=September 2000 |bibcode=2000KFNTS...3..213L}}</ref>

Many asteroids have [[Astronomy:Natural satellite|natural satellite]]s ([[Astronomy:Minor-planet moon|minor-planet moon]]s). {{As of|2021|10|df=US}}, there were 85 NEAs known to have at least one moon, including three known to have two moons.<ref>{{cite web |title=Asteroids with Satellites |publisher=Johnston's Archive |url=http://www.johnstonsarchive.net/astro/asteroidmoons.html |access-date=2018-03-17}}</ref> The asteroid [[Astronomy:3122 Florence|3122 Florence]], one of the largest potentially hazardous asteroids with a diameter of {{convert|4.5|km|mi|abbr=on}}, has two moons measuring {{convert|100–300|m|ft|abbr=on}} across, which were discovered by radar imaging during the asteroid's 2017 approach to Earth.<ref name="Florence-moons">{{cite news |author1=Lance Benner |author2=Shantanu Naidu |author3=Marina Brozovic |author4=Paul Chodas |title=Radar Reveals Two Moons Orbiting Asteroid Florence |work=News |publisher=NASA/JPL CNEOS |date=September 1, 2017 |url=https://cneos.jpl.nasa.gov/news/news199.html |access-date=2018-01-19 |url-status=live |archive-url=https://web.archive.org/web/20170903060914/https://cneos.jpl.nasa.gov/news/news199.html |archive-date=2017-09-03 }}</ref>

Near-Earth asteroids are divided into groups based on their semi-major axis (a), [[Astronomy:Apsis|perihelion]] distance (q), and [[Astronomy:Apsis|aphelion]] distance (Q):<ref name="NEO-groups">{{cite web |title=NEO Basics. NEO Groups |publisher=NASA/JPL CNEOS |url=http://cneos.jpl.nasa.gov/neo/groups.html |access-date=2017-11-09}}</ref><ref name="MorbidelliAstIII">{{cite journal |url=http://www.boulder.swri.edu/~bottke/Reprints/Morbidelli-etal_2002_AstIII_NEOs.pdf |title=Origin and Evolution of Near-Earth Objects |first1=Alessandro |last1=Morbidelli |first2=William F. Jr. |last2=Bottke |first3=Christiane |last3=Froeschlé |first4=Patrick |last4=Michel |journal=Asteroids III |editor=W. F. Bottke Jr. |editor2=A. Cellino |editor3=P. Paolicchi |editor4=R. P. Binzel |pages=409–422 |date=January 2002 |doi=10.2307/j.ctv1v7zdn4.33 |bibcode=2002aste.book..409M |access-date=2017-11-09 |url-status=live |archive-url=https://web.archive.org/web/20170809014123/http://www.boulder.swri.edu/%7Ebottke/Reprints/Morbidelli-etal_2002_AstIII_NEOs.pdf |archive-date=2017-08-09 }}</ref>
* The ''[[Astronomy:Atira asteroid|Atiras]]'' or ''Apoheles'' have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983&nbsp;AU). That is, {{nowrap|Q < 0.983 AU}}, which implies that the asteroid's semi-major axis is also less than 0.983 AU.<ref name="atiras">{{cite journal
|last1=de la Fuente Marcos |first1=Carlos
|last2=de la Fuente Marcos |first2=Raúl
|date=1 August 2019
|title=Understanding the evolution of Atira-class asteroid 2019 AQ<sub>3</sub>, a major step towards the future discovery of the Vatira population
|journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]]
|volume= 487
|issue= 2
|pages= 2742–2752
|arxiv=1905.08695
|bibcode=2019MNRAS.487.2742D
|doi=10.1093/mnras/stz1437|s2cid=160009327
}}</ref>
* The ''[[Astronomy:Aten asteroid|Atens]]'' have a semi-major axis of less than 1&nbsp;AU and cross Earth's orbit. Mathematically, {{nowrap|a < 1.0 AU}} and {{nowrap|Q > 0.983 AU}}. (0.983 AU is Earth's perihelion distance.)
* The ''[[Astronomy:Apollo asteroid|Apollos]]'' have a semi-major axis of more than 1&nbsp;AU and cross Earth's orbit. Mathematically, {{nowrap|a > 1.0 AU}} and {{nowrap|q < 1.017 AU}}. (1.017&nbsp;AU is Earth's aphelion distance.)
* The ''[[Astronomy:Amor asteroid|Amors]]'' have orbits strictly outside Earth's orbit: an Amor asteroid's perihelion distance (q) is greater than Earth's aphelion distance (1.017&nbsp;AU). Amor asteroids are also near-earth objects so {{nowrap|q < 1.3 AU}}. In summary, {{nowrap|1.017 AU < q < 1.3 AU}}. (This implies that the asteroid's semi-major axis (a) is also larger than 1.017&nbsp;AU.) Some Amor asteroid orbits cross the orbit of Mars.

=== Martian moons ===
{{Main|Astronomy:Moons of Mars|Astronomy:Phobos (moon)|Astronomy:Deimos (moon)}}
{{Multiple image
| direction         = horizontal
| total_width       = 300px
| image1            = Phobos_colour_2008.jpg
| caption1          = Phobos
| image2            = Deimos-MRO.jpg
| caption2          = Deimos
| align             = 
}}

It is unclear whether Martian moons Phobos and Deimos are captured asteroids or were formed due to impact event on Mars.<ref name="burns">Burns, Joseph A. (1992). "Contradictory Clues as to the Origin of the Martian Moons" in ''Mars'', H. H. Kieffer et al., eds., Tucson: University of Arizona Press, Tucson {{page needed|date=October 2022}}</ref> Phobos and Deimos both have much in common with carbonaceous [[Astronomy:C-type asteroid|C-type asteroid]]s, with [[Physics:Electromagnetic spectrum|spectra]], [[Earth:Albedo|albedo]], and [[Physics:Density|density]] very similar to those of C- or D-type asteroids.<ref name="c-type">{{cite web |url=https://www.nasa.gov/mission_pages/MRO/multimedia/20071127-caption.html |title=Views of Phobos and Deimos |work=[[Organization:NASA|NASA]] |date=27 November 2007 |access-date=19 July 2021 |archive-date=4 May 2016 |archive-url=https://web.archive.org/web/20160504234224/http://www.nasa.gov/mission_pages/MRO/multimedia/20071127-caption.html |url-status=dead }}</ref> Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids.<ref>{{cite web |title=Close Inspection for Phobos |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31031 |quote=One idea is that Phobos and Deimos, Mars's other moon, are captured asteroids. }}</ref><ref name="landis">Landis, Geoffrey A.; "Origin of Martian Moons from Binary Asteroid Dissociation", ''American Association for the Advancement of Science Annual Meeting''; Boston, MA, 2001, [https://ntrs.nasa.gov/search.jsp?R=946501&id=8&qs=No%3D70&N%3D4294808501 abstract]</ref> Both moons have very circular orbits which lie almost exactly in Mars's equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most probably by a combination of atmospheric drag and [[Physics:Tidal force|tidal force]]s,<ref name="cazenave">{{Cite journal |last1=Cazenave |first1=Anny |last2=Dobrovolskis |first2=Anthony R. |last3=Lago |first3=Bernard |date=1980 |title=Orbital history of the Martian satellites with inferences on their origin |journal=Icarus |volume=44 |issue=3 |pages=730–744 |doi=10.1016/0019-1035(80)90140-2 |bibcode=1980Icar...44..730C }}</ref> although it is not clear whether sufficient time was available for this to occur for Deimos.<ref name="burns" /> Capture also requires dissipation of energy. The current Martian atmosphere is too thin to capture a Phobos-sized object by atmospheric braking.<ref name="burns" /> [[Biography:Geoffrey A. Landis|Geoffrey A. Landis]] has pointed out that the capture could have occurred if the original body was a [[Astronomy:Binary asteroid|binary asteroid]] that separated under tidal forces.<ref name="landis" /><ref>{{cite journal | last = Canup | first = Robin | author-link = Robin Canup | title = Origin of Phobos and Deimos by the impact of a Vesta-to-Ceres sized body with Mars | date = 2018-04-18 | journal = Science Advances | volume = 4 | issue = 4 | page= eaar6887 | doi = 10.1126/sciadv.aar6887| pmid = 29675470 | pmc = 5906076 | bibcode = 2018SciA....4.6887C | doi-access = free }}</ref>

Phobos could be a second-generation Solar System object that [[Physics:Accretion (astrophysics)|coalesced]] in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars.<ref name="ESA2010">{{cite web |first1=Martin |last1=Pätzold |first2=Olivier |last2=Witasse |name-list-style=amp |url=https://www.esa.int/Science_Exploration/Space_Science/Mars_Express/Phobos_flyby_success |title=Phobos Flyby Success |publisher=ESA |date=4 March 2010 |access-date=4 March 2010}}</ref>

Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large [[Astronomy:Planetesimal|planetesimal]].<ref name="Craddock">Craddock, Robert A.; (1994); "The Origin of Phobos and Deimos", ''Abstracts of the 25th Annual Lunar and Planetary Science Conference, held in Houston, TX, 14–18 March 1994'', p. 293</ref> The high porosity of the interior of Phobos (based on the density of 1.88 g/cm<sup>3</sup>, voids are estimated to comprise 25 to 35 percent of Phobos's volume) is inconsistent with an asteroidal origin.<ref name="Andert">{{Cite journal |last1=Andert |first1=Thomas P. |display-authors=4 |last2=Rosenblatt |first2=Pascal |last3=Pätzold |first3=Martin |last4=Häusler |first4=Bernd |last5=Dehant |first5=Véronique M. |last6=Tyler |first6=George Leonard |last7=Marty |first7=Jean-Charles |title=Precise mass determination and the nature of Phobos |journal=[[Earth:Geophysical Research Letters|Geophysical Research Letters]] |volume=37 |issue=9 |page=L09202 |date = 7 May 2010 |doi=10.1029/2009GL041829 |bibcode=2010GeoRL..37.9202A |doi-access=free }}</ref> Observations of Phobos in the thermal infrared suggest a composition containing mainly phyllosilicates, which are well known from the surface of Mars. The spectra are distinct from those of all classes of [[Astronomy:Chondrite|chondrite]] meteorites, again pointing away from an asteroidal origin.<ref name="Giuranna">{{Cite conference |first1=Marco |last1=Giuranna |display-authors=4 |last2=Roush |first2=Ted L. |last3=Duxbury |first3=Thomas |last4=Hogan |first4=Robert C. |last5=Geminale |first5=Anna |last6=Formisano |first6=Vittorio |title=Compositional Interpretation of PFS/MEx and TES/MGS Thermal Infrared Spectra of Phobos |book-title=European Planetary Science Congress Abstracts, Vol. 5 |date=2010 |url=http://meetingorganizer.copernicus.org/EPSC2010/EPSC2010-211.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://meetingorganizer.copernicus.org/EPSC2010/EPSC2010-211.pdf |archive-date=2022-10-09 |url-status=live |access-date=1 October 2010 }}</ref> Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit,<ref name="Blast">{{cite web |url=https://www.space.com/9201-mars-moon-phobos-forged-catastrophic-blast.html |title=Mars Moon Phobos Likely Forged by Catastrophic Blast |work=Space.com |date=27 September 2010 |access-date=1 October 2010}}</ref> similar to the prevailing theory for the origin of Earth's moon.

== Characteristics ==

=== Size distribution ===
[[File:Asteroids by size and number.svg|thumb|The asteroids of the Solar System, categorized by size and number|300x300px]]
{{image frame
|width=256
 | content = {{Graph:Chart
  | width=75
  | height=75
  | type=pie
  | legend=
  | x=Ceres,Vesta,Pallas,Hygiea,Interamnia,Eunomia,other
  | y1=938,259,204,87,35,30,841<!--total 2394x18kg-->
  | showValues=angle:0,format:.0f
          }}
 | caption = The masses of the largest asteroids in the main belt: [[Astronomy:1 Ceres|1 Ceres]] (blue), [[Astronomy:4 Vesta|4 Vesta]], [[Astronomy:2 Pallas|2 Pallas]], [[Astronomy:10 Hygiea|10 Hygiea]], [[Astronomy:704 Interamnia|704 Interamnia]], [[Astronomy:15 Eunomia|15 Eunomia]] and the remainder of the Main Belt (pink). The unit of mass is {{e|18}} kg.}}
Asteroids vary greatly in size, from almost {{val|1000|u=km}} for the largest down to rocks just 1&nbsp;meter across, below which an object is classified as a [[Astronomy:Meteoroid|meteoroid]].{{efn|The definition in the 1995 paper (Beech and Steel) has been updated by a 2010 paper (Rubin and Grossman) and the discovery of 1&nbsp;meter asteroids.}} The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors,<ref name=Schmidt2007>{{cite journal |title=Hubble Space Telescope Observations of 2&nbsp;Pallas |journal=Bulletin of the American Astronomical Society |volume=39 |page=485 |date=2007 |display-authors=6 |last1=Schmidt |first1= B. |last2=Russell |first2= C.T. |last3=Bauer |first3= J.M. |last4=Li |first4= J. |last5=McFadden |first5= L.A. |last6=Mutchler |first6= M. |last7=Parker |first7= J.W. |last8=Rivkin |first8= A.S. |last9=Stern |first9= S.A. |author10=Thomas, P.C. |bibcode=2007DPS....39.3519S}}</ref> and are thought to be surviving [[Astronomy:Protoplanet|protoplanet]]s. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either battered [[Astronomy:Planetesimal|planetesimal]]s or fragments of larger bodies.

The [[Astronomy:Dwarf planet|dwarf planet]] [[Astronomy:Ceres (dwarf planet)|Ceres]] is by far the largest asteroid, with a diameter of {{cvt|940|km|-1}}. The next largest are [[Astronomy:4 Vesta|4 Vesta]] and [[Astronomy:2 Pallas|2 Pallas]], both with diameters of just over {{cvt|500|km|-2}}. Vesta is the brightest of the four main-belt asteroids that can, on occasion, be visible to the naked eye.<ref>{{cite book | title=The Observer's Guide to Astronomy | volume=1 | series=Practical Astronomy Handbooks | editor-first=Patrick | editor-last=Martinez | translator-last1=Dunlop | translator-first1=Storm | publisher=Cambridge University Press | date=1994 | isbn=978-0-521-37945-8 | page=297 | url=https://books.google.com/books?id=k5iUVz7iFTQC&pg=PA297 }}</ref> On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see [[Astronomy:99942 Apophis|99942 Apophis]].

The mass of all the objects of the [[Astronomy:Asteroid belt|asteroid belt]], lying between the orbits of [[Astronomy:Mars|Mars]] and [[Astronomy:Jupiter|Jupiter]], is estimated to be {{val|2394|6|e=18|u=kg}}, ≈&thinsp;3.25% of the mass of the Moon. Of this, [[Astronomy:Ceres (dwarf planet)|Ceres]] comprises {{val|938|e=18|u=kg}}, about 40% of the total. Adding in the next three most massive objects, [[Astronomy:4 Vesta|Vesta]] (11%), [[Astronomy:2 Pallas|Pallas]] (8.5%), and [[Astronomy:10 Hygiea|Hygiea]] (3–4%), brings this figure up to a bit over 60%, whereas the next seven most-massive asteroids bring the total up to 70%.<ref name="Pitjeva2018"/> The number of asteroids increases rapidly as their individual masses decrease.

The number of asteroids decreases markedly with increasing size. Although the size distribution generally follows a [[Power law|power law]], there are 'bumps' at about {{val|5|u=km}} and {{val|100|u=km}}, where more asteroids than expected from such a curve are found. Most asteroids larger than approximately 120&nbsp;km in diameter are primordial (surviving from the accretion epoch), whereas most smaller asteroids are products of fragmentation of primordial asteroids. The primordial population of the main belt was probably 200 times what it is today.<ref>{{Cite journal |last1=Bottkejr |first1=W |last2=Durda |first2=D |last3=Nesvorny |first3=D |last4=Jedicke |first4=R |last5=Morbidelli |first5=A |last6=Vokrouhlicky |first6=D |last7=Levison |first7=H |date=May 2005 |title=The fossilized size distribution of the main asteroid belt |url=https://linkinghub.elsevier.com/retrieve/pii/S0019103504003811 |journal=Icarus |language=en |volume=175 |issue=1 |pages=111–140 |doi=10.1016/j.icarus.2004.10.026|bibcode=2005Icar..175..111B }}</ref><ref>{{Cite journal |last1=O'Brien |first1=David P. |last2=Sykes |first2=Mark V. |date=December 2011 |title=The Origin and Evolution of the Asteroid Belt{{snd}}Implications for Vesta and Ceres |url=http://link.springer.com/10.1007/s11214-011-9808-6 |journal=Space Science Reviews |language=en |volume=163 |issue=1–4 |pages=41–61 |doi=10.1007/s11214-011-9808-6 |bibcode=2011SSRv..163...41O |s2cid=121856071 |issn=0038-6308}}</ref>

====Largest asteroids====

{{Multiple image
| direction         = vertical
| image1            = 42 of the largest objects in the asteroid belt.jpg
| caption1          = 42 of the largest objects in the asteroid belt captured by [[Organization:European Southern Observatory|ESO]]'s [[Astronomy:Very Large Telescope|Very Large Telescope]]
| image2            = Eros, Vesta and Ceres size comparison.jpg
| caption2          = Eros, Vesta and Ceres size comparison
| total_width       = 250
}}

Three largest objects in the asteroid belt, [[Astronomy:Ceres (dwarf planet)|Ceres]], [[Astronomy:4 Vesta|Vesta]], and [[Astronomy:2 Pallas|Pallas]], are intact [[Astronomy:Protoplanet|protoplanet]]s that share many characteristics common to planets, and are atypical compared to the majority of irregularly shaped asteroids. The fourth-largest asteroid, [[Astronomy:10 Hygiea|Hygiea]], appears nearly spherical although it may have an undifferentiated interior,<ref>{{Cite web|title=Asteroids {{!}} Imaging the Universe|url=http://astro.physics.uiowa.edu/ITU/labs/general-astronomy/asteroids/|access-date=2021-08-31|website=astro.physics.uiowa.edu|archive-date=31 August 2021|archive-url=https://web.archive.org/web/20210831200522/http://astro.physics.uiowa.edu/ITU/labs/general-astronomy/asteroids/|url-status=dead}}</ref> like the majority of asteroids. The four largest asteroids constitute half the mass of the asteroid belt.

Ceres is the only asteroid that appears to have a [[Physics:Plasticity|plastic]] shape under its own gravity and hence the only one that is a [[Astronomy:Dwarf planet|dwarf planet]].<ref name=IAU-2006/> It has a much higher [[Astronomy:Absolute magnitude#Solar System bodies (H)|absolute magnitude]] than the other asteroids, of around 3.32,<ref name=AstJ-2002-v123-p549/> and may possess a surface layer of ice.<ref name="planetary"/> Like the planets, Ceres is differentiated: it has a crust, a mantle and a core.<ref name="planetary"/> No meteorites from Ceres have been found on Earth.<ref name=satellites/>

Vesta, too, has a differentiated interior, though it formed inside the Solar System's [[Physics:Frost line (astrophysics)|frost line]], and so is devoid of water;<ref>{{cite press release |title=Asteroid or mini-planet? Hubble maps the ancient surface of Vesta |date=19 April 1995 |id=STScI-1995-20 |url=http://hubblesite.org/news_release/news/1995-20 |website=Hubble Space Telescope |publisher=Space Telescope Science Institute |access-date=16 December 2017}}<br />{{cite press release |title=Key stages in the evolution of the asteroid Vesta |website=Hubble Space Telescope |publisher=Space Telescope Science Institute |date=19 April 1995 |url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/20/image/c |access-date=20 October 2007 |url-status=live |archive-url=https://web.archive.org/web/20080907192327/http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/20/image/c |archive-date=7 September 2008}}</ref><ref>
{{cite journal |last1=Russel |first1=C. |last2=Raymond |first2=C. |last3=Fraschetti |first3=T. |last4=Rayman |first4=M. |last5=Polanskey |first5=C. |last6=Schimmels |first6=K. |last7=Joy |first7=S. |year=2005 |title=Dawn mission and operations |journal=Proceedings of the International Astronomical Union |volume=1 |issue=S229 |pages=97–119 |bibcode=2006IAUS..229...97R |doi=10.1017/S1743921305006691 |doi-access=free }}</ref> its composition is mainly of basaltic rock with minerals such as olivine.<ref>{{cite journal |last=Burbine |first=T.H. |date=July 1994 |title=Where are the olivine asteroids in the main belt? |journal=Meteoritics |volume=29 |issue=4 |page=453 |bibcode-access=free |bibcode=1994Metic..29..453B}}</ref> Aside from the large crater at its southern pole, Rheasilvia, Vesta also has an ellipsoidal shape. Vesta is the parent body of the Vestian family and other [[Astronomy:V-type asteroid|V-type asteroid]]s, and is the source of the [[Astronomy:HED meteorite|HED meteorite]]s, which constitute 5% of all meteorites on Earth.

Pallas is unusual in that, like [[Astronomy:Uranus|Uranus]], it rotates on its side, with its axis of rotation tilted at high angles to its orbital plane.<ref name="Torppa1996"/> Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated.<ref name=Icarus-1983-v56-p398/> Pallas is the parent body of the Palladian family of asteroids.

Hygiea is the largest carbonaceous asteroid<ref name=Icarus-2002-156-p202/> and, unlike the other largest asteroids, lies relatively close to the plane of the ecliptic. It is the largest member and presumed parent body of the Hygiean family of asteroids. Because there is no sufficiently large crater on the surface to be the source of that family, as there is on Vesta, it is thought that Hygiea may have been completely disrupted in the collision that formed the Hygiean family and recoalesced after losing a bit less than 2% of its mass. Observations taken with the [[Astronomy:Very Large Telescope|Very Large Telescope]]'s SPHERE imager in 2017 and 2018, revealed that Hygiea has a nearly spherical shape, which is consistent both with it being in [[Astronomy:Hydrostatic equilibrium|hydrostatic equilibrium]], or formerly being in hydrostatic equilibrium, or with being disrupted and recoalescing.<ref name=NatAstr-2019-10-28/><ref name=Strickland2019/>

Internal differentiation of large asteroids is possibly related to their lack of [[Astronomy:Natural satellite|natural satellite]]s, as satellites of main belt asteroids are mostly believed to form from collisional disruption, creating a [[Astronomy:Rubble pile|rubble pile]] structure.<ref name=satellites>{{cite journal|title=Dawn mission's search for satellites of Ceres: Intact protoplanets don't have satellites|journal=Icarus|volume=316|date=December 2018|pages=191–204|author1-first=Lucy A.|author1-last=McFadden |author2-first=David R. |author2-last=Skillman |author3-first=N |author3-last=Memarsadeghi |doi=10.1016/j.icarus.2018.02.017|bibcode=2018Icar..316..191M |s2cid=125181684 |quote=Examination of the physical properties of the 41 largest and most massive main belt asteroids suggests that large asteroids without satellites are intact and their interiors have internal strength. This is consistent with results from the Dawn mission at both Vesta and Ceres. Ceres' volatile-rich composition also is a likely contributor to both the absence of satellites at Ceres and of Ceres meteorites at Earth. These results suggest that collisional disruption creating rubble pile structure is a necessary condition for formation of satellites around main belt asteroids.}}</ref>

{| class="wikitable"
|+ Attributes of largest asteroids
|- style="font-size: smaller;"
!Name
!Orbital<br />radius<br />([[Astronomy:Astronomical unit|AU]])
![[Astronomy:Orbital period|Orbital<br />period]]<br />(years)
!Inclination<br />to ecliptic
![[Astronomy:Orbital eccentricity|Orbital<br />eccentricity]]
! Diameter<br />(km)
! Diameter<br />(% of [[Astronomy:Moon|Moon]])
! Mass<br />({{e|18}} kg)
! Mass<br />(% of Ceres)
! Density<br />(g/cm<sup>3</sup>)
! Rotation<br />period<br />(hr)
|- style="text-align:center;"
! style="text-align:left;"| [[Astronomy:Ceres (dwarf planet)|Ceres]]
| 2.77
| 4.60
| 10.6°
| 0.079
| 964×964×892<br />(mean 939.4)
| 27%
| 938
| 100%
| 2.16±0.01
| 9.07
|- style="text-align:center;"
! style="text-align:left;"| [[Astronomy:4 Vesta|Vesta]]
| 2.36
| 3.63
| 7.1°
| 0.089
| 573×557×446<br />(mean 525.4)
| 15%
| 259
| 28%
| 3.46 ± 0.04
| 5.34
|- style="text-align:center;"
! style="text-align:left;"| [[Astronomy:2 Pallas|Pallas]]
| 2.77
| 4.62
| 34.8°
| 0.231
| 550×516×476<br />(mean 511±4)
| 15%
| 204±3
| 21%
| 2.92±0.08
| 7.81
|- style="text-align:center;"
! style="text-align:left;"| [[Astronomy:10 Hygiea|Hygiea]]
| 3.14
| 5.56
| 3.8°
| 0.117
| 450×430×424<br />(mean 433±8)
| 12%
| 87±7
| 9%
| 2.06±0.20
| 13.8
|}

=== Rotation ===
{{Further|Astronomy:List of fast rotators (minor planets)|Astronomy:List of slow rotators (minor planets)}}
Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. Very few asteroids with a diameter larger than 100 meters have a rotation period less than 2.2&nbsp;hours.<ref>{{cite web |title=About Lightcurves |series=Asteroid Lightcurve Photometry Database |website=ALCDEF |date=4 December 2018 |url=http://alcdef.org/ |access-date=27 December 2018}}</ref> For asteroids rotating faster than approximately this rate, the inertial force at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that most asteroids with a diameter over 100 meters are [[Astronomy:Rubble pile|rubble pile]]s formed through the accumulation of debris after collisions between asteroids.<ref name=Rossi-2004/>

=== Color ===
Asteroids become darker and redder with age due to [[Astronomy:Space weathering|space weathering]].<ref name="UHi2005-05-19" /> However evidence suggests most of the color change occurs rapidly, in the first hundred thousand years, limiting the usefulness of spectral measurement for determining the age of asteroids.<ref name="Courtland-2009" />

=== Surface features ===
[[File:Vesta Cratered terrain with hills and ridges.jpg|thumb|Cratered terrain on 4 Vesta]]

Except for the "[[Astronomy:List of exceptional asteroids#Largest by mass|big four]]" (Ceres, Pallas, Vesta, and Hygiea), asteroids are likely to be broadly similar in appearance, if irregular in shape. {{convert|50|km|mi|abbr=on}} [[Astronomy:253 Mathilde|253 Mathilde]] is a rubble pile saturated with craters with diameters the size of the asteroid's radius. Earth-based observations of {{convert|300|km|mi|abbr=on}} [[Astronomy:511 Davida|511 Davida]], one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters.<ref name="Icarus-2007-v191-p616" /> Medium-sized asteroids such as Mathilde and [[Astronomy:243 Ida|243 Ida]], that have been observed up close, also reveal a deep regolith covering the surface. Of the big four, Pallas and Hygiea are practically unknown. Vesta has compression fractures encircling a radius-size crater at its south pole but is otherwise a [[Spheroid|spheroid]].

''[[Engineering:Dawn (spacecraft)|Dawn spacecraft]]'' revealed that Ceres has a heavily cratered surface, but with fewer large craters than expected.<ref name="marchi">{{cite journal |last1=Marchi |first1=S. |last2=Ermakov |first2=A. I. |last3=Raymond |first3=C. A. |last4=Fu |first4=R. R. |last5=O'Brien |first5=D. P. |last6=Bland |first6=M. T. |last7=Ammannito |first7=E. |last8=De Sanctis |first8=M. C. |last9=Bowling |first9=T. |last10=Schenk |first10=P. |last11=Scully |first11=J. E. C. |date=26 July 2016 |title=The missing large impact craters on Ceres |journal=Nature Communications |volume=7 |pages=12257 |bibcode=2016NatCo...712257M |doi=10.1038/ncomms12257 |pmc=4963536 |pmid=27459197 |last12=Buczkowski |first12=D. L. |last13=Williams |first13=D. A. |last14=Hiesinger |first14=H. |last15=Russell |first15=C. T.}}</ref> Models based on the formation of the current asteroid belt had suggested Ceres should possess 10 to 15 craters larger than {{convert|400|km|mi|abbr=on}} in diameter.<ref name="marchi" /> The largest confirmed crater on Ceres, Kerwan Basin, is {{convert|284|km|mi|abbr=on}} across.<ref>{{cite journal |last=David A. Williams |first= T. Kneiss |date=December 2018 |title=The geology of the Kerwan quadrangle of dwarf planet Ceres: Investigating Ceres' oldest, largest impact basin |url=https://www.sciencedirect.com/science/article/abs/pii/S0019103516305632 |url-status=live |journal=Icarus |volume=316 |pages=99–113 |bibcode=2018Icar..316...99W |doi=10.1016/j.icarus.2017.08.015 |archive-url=https://web.archive.org/web/20210816123323/https://www.sciencedirect.com/science/article/abs/pii/S0019103516305632?via%3Dihub |archive-date=16 August 2021 |access-date=16 August 2021 |s2cid=85539501}}</ref> The most likely reason for this is [[Physics:Viscoelasticity|viscous relaxation]] of the crust slowly flattening out larger impacts.<ref name="marchi" />

=== Composition ===
Asteroids are classified by their characteristic [[Physics:Emission spectrum|emission spectra]], with the majority falling into three main groups: [[Astronomy:C-type asteroid|C-type]], [[Astronomy:M-type asteroid|M-type]], and [[Astronomy:S-type asteroid|S-type]]. These were named after and are generally identified with carbonaceous ([[Chemistry:Carbon|carbon-rich]]), [[Chemistry:Metal|metal]]lic, and [[Chemistry:Silica|silica]]ceous (stony) compositions, respectively. The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle, where Vesta is thought to have a nickel-iron core, [[Chemistry:Olivine|olivine]] mantle, and basaltic crust.<ref name="Hubble-Vespa-1995-04-19" /> Thought to be the largest undifferentiated asteroid, [[Astronomy:10 Hygiea|10 Hygiea]] seems to have a uniformly primitive composition of carbonaceous chondrite, but it may actually be a differentiated asteroid that was globally disrupted by an impact and then reassembled. Other asteroids appear to be the remnant cores or mantles of proto-planets, high in rock and metal. Most small asteroids are believed to be piles of rubble held together loosely by gravity, although the largest are probably solid. Some asteroids have moons or are co-orbiting [[Astronomy:Binary asteroid|binaries]]: rubble piles, moons, binaries, and scattered [[Astronomy:Asteroid family|asteroid families]] are thought to be the results of collisions that disrupted a parent asteroid, or possibly a [[Astronomy:Disrupted planet|planet]].<ref name="ARX-20060816" />

In the main asteroid belt, there appear to be two primary populations of asteroid: a dark, volatile-rich population, consisting of the [[Astronomy:C-type asteroid|C-type]] and [[Astronomy:P-type asteroid|P-type]] asteroids, with albedos less than 0.10 and densities under {{val|2.2|u=g/cm3}}, and a dense, volatile-poor population, consisting of the [[Astronomy:S-type asteroid|S-type]] and [[Astronomy:M-type asteroid|M-type]] asteroids, with albedos over 0.15 and densities greater than 2.7. Within these populations, larger asteroids are denser, presumably due to compression. There appears to be minimal macro-porosity (interstitial vacuum) in the score of asteroids with masses greater than {{val|10|e=18|u=kg}}.<ref name="VLT">P. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. ''Astronomy & Astrophysics'' 54, A56</ref>

Composition is calculated from three primary sources: [[Earth:Albedo|albedo]], surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume. The investigated asteroids are as large as 280&nbsp;km in diameter, and include [[Astronomy:121 Hermione|121 Hermione]] (268×186×183&nbsp;km), and [[Astronomy:87 Sylvia|87 Sylvia]] (384×262×232&nbsp;km). Few asteroids are larger than 87&nbsp;Sylvia, none of them have moons. The fact that such large asteroids as Sylvia may be rubble piles, presumably due to disruptive impacts, has important consequences for the formation of the Solar System: computer simulations of collisions involving solid bodies show them destroying each other as often as merging, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly.<ref name="Icarus-2011-02-p1022" />

==== Water ====
{{Main|Astronomy:Asteroidal water}}

Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the [[Astronomy:Moon|Moon]].<ref name="Campins2010" /> In 2009, the presence of [[Earth:Ice|water ice]] was confirmed on the surface of [[Astronomy:24 Themis|24 Themis]] using NASA's Infrared Telescope Facility. The surface of the asteroid appears completely covered in ice. As this ice layer is [[Sublimation (phase transition)|sublimating]], it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface.<ref name="Cowen-2009" /><ref name="Atkinson-2009" /><ref name="Campins2010" /><ref name="RivkinEmery2010" /> The presence of ice on 24 Themis makes the initial theory plausible.<ref name="Campins2010" />

In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the [[Astronomy:White dwarf|white dwarf]] [[Astronomy:GD 61|GD 61]].<ref>{{Cite web|url=https://keckobservatory.org/watery_asteroid_discovered_in_dying_star_points_to_habitable_exoplanets/|title=Watery Asteroid Discovered in Dying Star Points to Habitable Exoplanets – W. M. Keck Observatory}}</ref> On 22&nbsp;January 2014, [[Organization:European Space Agency|European Space Agency]] (ESA) scientists reported the detection, for the first definitive time, of [[Physics:Water vapor|water vapor]] on [[Astronomy:Ceres (dwarf planet)|Ceres]], the largest object in the asteroid belt.<ref name="KüppersO'Rourke2014" /> The detection was made by using the [[Astronomy:Far-infrared astronomy|far-infrared abilities]] of the [[Astronomy:Herschel Space Observatory|Herschel Space Observatory]].<ref name="NASA-20140122" /> The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."<ref name="NASA-20140122" />

Findings have shown that [[Astronomy:Solar wind|solar wind]]s can react with the oxygen in the upper layer of the asteroids and create water. It has been estimated that "every cubic metre of irradiated rock could contain up to 20 litres"; study was conducted using an atom probe tomography, numbers are given for the Itokawa S-type asteroid.<ref>{{cite journal |last1=Daly |first1=Luke |last2=Lee |first2=Martin R. |last3=Hallis |first3=Lydia J. |last4=Ishii |first4=Hope A. |last5=Bradley |first5=John P. |last6=Bland |first6=Phillip A. |last7=Saxey |first7=David W. |last8=Fougerouse |first8=Denis |last9=Rickard |first9=William D. A. |last10=Forman |first10=Lucy V. |last11=Timms |first11=Nicholas E. |last12=Jourdan |first12=Fred |last13=Reddy |first13=Steven M. |last14=Salge |first14=Tobias |last15=Quadir |first15=Zakaria |last16=Christou |first16=Evangelos |last17=Cox |first17=Morgan A. |last18=Aguiar |first18=Jeffrey A. |last19=Hattar |first19=Khalid |last20=Monterrosa |first20=Anthony |last21=Keller |first21=Lindsay P. |last22=Christoffersen |first22=Roy |last23=Dukes |first23=Catherine A. |last24=Loeffler |first24=Mark J. |last25=Thompson |first25=Michelle S. |title=Solar wind contributions to Earth's oceans |journal=Nature Astronomy |date=December 2021 |volume=5 |issue=12 |pages=1275–1285 |doi=10.1038/s41550-021-01487-w |bibcode=2021NatAs...5.1275D |osti=1834330 |s2cid=244744492 |url=https://www.nature.com/articles/s41550-021-01487-w |access-date=30 March 2022 |language=en |issn=2397-3366}}</ref><ref>{{cite web |title=Earth's water may have been formed by solar winds |url=https://www.nhm.ac.uk/discover/news/2021/december/earth-s-water-may-have-been-formed-by-solar-winds.html |website=nhm.ac.uk |access-date=30 March 2022 |language=en}}</ref>

Acfer 049, a meteorite discovered in Algeria in 1990, was shown in 2019 to have an ultraporous lithology (UPL): porous texture that could be formed by removal of ice that filled these pores, this suggests that UPL "represent fossils of primordial ice".<ref>{{cite journal |last1=Matsumoto |first1=Megumi |last2=Tsuchiyama |first2=Akira |last3=Nakato |first3=Aiko |last4=Matsuno |first4=Junya |last5=Miyake |first5=Akira |last6=Kataoka |first6=Akimasa |last7=Ito |first7=Motoo |last8=Tomioka |first8=Naotaka |last9=Kodama |first9=Yu |last10=Uesugi |first10=Kentaro |last11=Takeuchi |first11=Akihisa |last12=Nakano |first12=Tsukasa |last13=Vaccaro |first13=Epifanio |title=Discovery of fossil asteroidal ice in primitive meteorite Acfer 094 |journal=Science Advances |date=November 2019 |volume=5 |issue=11 |pages=eaax5078 |doi=10.1126/sciadv.aax5078|pmid=31799392 |pmc=6867873 |bibcode=2019SciA....5.5078M }}</ref>

==== Organic compounds ====

Asteroids contain traces of [[Chemistry:Amino acid|amino acid]]s and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth (an event called "[[Biology:Panspermia|panspermia]]").<ref name="SPACE-2001-12-19" /><ref name="Reuell-2019" /> In August&nbsp;2011, a report, based on [[Organization:NASA|NASA]] studies with [[Astronomy:Meteorite|meteorite]]s found on [[Astronomy:Earth|Earth]], was published suggesting [[Biology:DNA|DNA]] and [[Biology:RNA|RNA]] components ([[Chemistry:Adenine|adenine]], [[Chemistry:Guanine|guanine]] and related organic molecules) may have been formed on asteroids and [[Astronomy:Comet|comet]]s in [[Astronomy:Outer space|outer space]].<ref name="Callahan" /><ref name="Steigerwald" /><ref name="DNA" />

In November 2019, scientists reported detecting, for the first time, [[Chemistry:Sugar|sugar molecules]], including [[Chemistry:Ribose|ribose]], in [[Astronomy:Meteorite|meteorite]]s, suggesting that chemical processes on asteroids can produce some fundamentally essential bio-ingredients important to life, and supporting the notion of an [[Biology:RNA world|RNA world]] prior to a DNA-based [[Biology:Abiogenesis|origin of life]] on Earth, and possibly, as well, the notion of [[Biology:Panspermia|panspermia]].<ref name="NASA-20191118" /><ref name="PNAS-20191118" /><ref>{{Cite web |last=Steigerwald |first=Bill |date=2022-03-31 |title=Could the Blueprint for Life Have Been Generated in Asteroids? |url=http://www.nasa.gov/feature/goddard/2022/life-blueprint-in-asteroids |access-date=2022-07-06 |website=NASA}}</ref>

== Classification ==
Asteroids are commonly categorized according to two criteria: the characteristics of their orbits, and features of their reflectance [[Physics:Visible spectrum|spectrum]].

=== Orbital classification ===
[[File:Animation_of_(419624)_2010_SO16_orbit.gif|thumb|A complex horseshoe orbit (the vertical looping is due to inclination of the smaller body's orbit to that of the Earth, and would be absent if both orbited in the same plane){{legend2|Yellow|Sun}}{{·}}{{legend2|RoyalBlue|Earth}}{{·}}{{legend2|Magenta|[[Astronomy:(419624) 2010 SO16|(419624) 2010 SO16]]}}]]
Many asteroids have been placed in groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past.<ref name=AstFams-Icarus-1995/> Families are more common and easier to identify within the main asteroid belt, but several small families have been reported among the [[Astronomy:Jupiter trojan|Jupiter trojan]]s.<ref name="JewittEtal2004"/> Main belt families were first recognized by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.

About 30–35% of the bodies in the asteroid belt belong to dynamical families, each thought to have a common origin in a past collision between asteroids. A family has also been associated with the plutoid [[Astronomy:Dwarf planet|dwarf planet]] {{dp|Haumea}}.

Some asteroids have unusual [[Astronomy:Horseshoe orbit|horseshoe orbit]]s that are co-orbital with [[Astronomy:Earth|Earth]] or another planet. Examples are [[Astronomy:3753 Cruithne|3753 Cruithne]] and {{mpl|2002 AA|29}}. The first instance of this type of orbital arrangement was discovered between [[Astronomy:Saturn|Saturn]]'s moons [[Astronomy:Epimetheus (moon)|Epimetheus]] and [[Astronomy:Janus (moon)|Janus]]. Sometimes these horseshoe objects temporarily become [[Astronomy:Quasi-satellite|quasi-satellite]]s for a few decades or a few hundred years, before returning to their earlier status. Both Earth and [[Astronomy:Venus|Venus]] are known to have quasi-satellites.

Such objects, if associated with Earth or Venus or even hypothetically [[Astronomy:Mercury (planet)|Mercury]], are a special class of [[Astronomy:Aten asteroid|Aten asteroid]]s. However, such objects could be associated with the outer planets as well.

=== Spectral classification ===
{{Main|Astronomy:Asteroid spectral types}}

In 1975, an asteroid [[Taxonomy (general)|taxonomic]] system based on [[Color|color]], [[Earth:Albedo|albedo]], and [[Physics:Spectral line|spectral shape]] was developed by Chapman, [[Biography:David Morrison (astrophysicist)|Morrison]], and Zellner.<ref name=CMZ-1975-Icarus/> These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: [[Astronomy:C-type asteroid|C-types]] for dark carbonaceous objects (75% of known asteroids), [[Astronomy:S-type asteroid|S-types]] for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include many other asteroid types. The number of types continues to grow as more asteroids are studied.

The two most widely used taxonomies now used are the Tholen classification and SMASS classification. The former was proposed in 1984 by [[Biography:David J. Tholen|David J. Tholen]], and was based on data collected from an eight-color asteroid survey performed in the 1980s. This resulted in 14&nbsp;asteroid categories.<ref name=Tholen-1989/> In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24&nbsp;different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the [[Astronomy:M-type asteroid|M-type]]. There are also several smaller classes.<ref name=Bus-2002/>

The proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.

==== Problems ====
Originally, spectral designations were based on inferences of an asteroid's composition.<ref name=McSween-1999/> However, the correspondence between spectral class and composition is not always very good, and a variety of classifications are in use. This has led to significant confusion. Although asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of the same (or similar) materials.

=== Active asteroids ===

Active asteroids are objects that have asteroid-like orbits but show [[Astronomy:Comet|comet]]-like visual characteristics. That is, they show [[Astronomy:Coma (cometary)|comae]], [[Astronomy:Comet tail|tails]], or other visual evidence of mass-loss (like a comet), but their orbit remains within [[Astronomy:Jupiter|Jupiter]]'s orbit (like an asteroid).<ref name="Jewitt" /><ref name="JHA15">{{cite book|chapter=The Active Asteroids|first1=David|last1=Jewitt|first2=Henry|last2=Hsieh|first3=Jessica|last3=Agarwal|year=2015|title= Asteroids IV|pages=221–241| editor1-last =  Michel| editor1-first = P. | editor2-last = others| display-editors = 1 | publisher=[[Organization:University of Arizona|University of Arizona]]|doi= 10.2458/azu_uapress_9780816532131-ch012 |arxiv=1502.02361|bibcode=2015aste.book..221J|isbn=978-0-8165-3213-1|s2cid=119209764| chapter-url= http://www2.ess.ucla.edu/~jewitt/papers/2015/JHA15.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www2.ess.ucla.edu/~jewitt/papers/2015/JHA15.pdf |archive-date=2022-10-09 |url-status=live |access-date=2020-01-30}}</ref> These bodies were originally designated '''main-belt comets''' (MBCs) in 2006 by astronomers David Jewitt and Henry Hsieh, but this name implies they are necessarily icy in composition like a comet and that they only exist within the [[Astronomy:Asteroid belt|main-belt]], whereas the growing population of active asteroids shows that this is not always the case.<ref name="Jewitt">{{cite web |title=The Active Asteroids |publisher=UCLA, Department of Earth and Space Sciences |author=David Jewitt |url=http://www2.ess.ucla.edu/~jewitt/mbc.html |access-date=2020-01-26|author-link=David Jewitt }}</ref><ref name="NYT-20190319">{{cite news |last1=Chang |first1=Kenneth |last2=Stirone |first2=Shannon |title=The Asteroid Was Shooting Rocks Into Space. 'Were We Safe in Orbit?' – NASA's Osiris-Rex and Japan's Hayabusa2 spacecraft reached the space rocks they are surveying last year, and scientists from both teams announced early findings on Tuesday (03/19/2019) |url=https://www.nytimes.com/2019/03/19/science/bennu-ryugu-asteroids.html |date=19 March 2019 |work={{wipe|The New York Times}} |access-date=21 March 2019 }}</ref><ref>{{cite web |title=Hubble Observes Six Tails from an Unusual Asteroid|publisher=Space Telescope Science Institute (STScI), official YouTube channel for the Hubble Space Telescope|url=https://www.youtube.com/watch?v=CGgRNWUFfZ0  |archive-url=https://ghostarchive.org/varchive/youtube/20211222/CGgRNWUFfZ0 |archive-date=2021-12-22 |url-status=live|access-date=2014-11-15}}{{cbignore}}</ref>

The first active asteroid discovered is [[Astronomy:7968 Elst–Pizarro|7968 Elst–Pizarro]]. It was discovered (as an asteroid) in 1979 but then was found to have a tail by [[Biography:Eric Walter Elst|Eric Elst]] and Guido Pizarro in 1996 and given the cometary designation 133P/Elst-Pizarro.<ref name="Jewitt"/><ref name=HH133P>{{cite web|last=Hsieh|first=Henry|title=133P/Elst-Pizarro|url=http://www.ifa.hawaii.edu/~hsieh/elstpiz.shtml|publisher=UH Institute for Astronomy|access-date=22 June 2012|date=January 20, 2004|url-status=dead|archive-url=https://web.archive.org/web/20111026205338/http://www.ifa.hawaii.edu/~hsieh/elstpiz.shtml|archive-date=26 October 2011}}</ref> Another notable object is 311P/PanSTARRS: observations made by the [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]] revealed that it had six comet-like tails.<ref name="hubblesite">{{cite web|title=NASA's Hubble Sees Asteroid Spouting Six Comet-Like Tails|url=http://hubblesite.org/newscenter/archive/releases/2013/52/text/|publisher=Hubblesite|date=7 November 2013}}</ref> The tails are suspected to be streams of material ejected by the asteroid as a result of a [[Astronomy:Rubble pile|rubble pile]] asteroid spinning fast enough to remove material from it.<ref name=Jewitt2013>{{cite journal
 |last1=Jewitt |first1=D.
 |last2=Agarwal |first2=J.
 |last3=Weaver |first3=H.
 |last4=Mutchler |first4=M.
 |last5=Larson |first5=S.
 |year=2013
 |title=The Extraordinary Multi-Tailed Main-Belt Comet P/2013 P5
 |journal=[[Astronomy:The Astronomical Journal|The Astronomical Journal]]
 |volume= 778|issue= 1|pages=L21
 |arxiv=1311.1483
 |bibcode=2013ApJ...778L..21J
 |doi=10.1088/2041-8205/778/1/L21
|s2cid=67795816
 }}</ref>

[[File:Hubble view of Dimorphos ejecta (October 8).png|thumb|Dimorphos and the tail created after the DART impact, photo by the Hubble Space Telescope]]

By smashing into the asteroid [[Astronomy:Dimorphos|Dimorphos]], NASA's [[Astronomy:Double Asteroid Redirection Test|Double Asteroid Redirection Test]] spacecraft made it an active asteroid. Scientists had proposed that some active asteroids are the result of impact events, but no one had ever observed the activation of an asteroid. The DART mission activated Dimorphos under precisely known and carefully observed impact conditions, enabling the detailed study of the formation of an active asteroid for the first time.<ref name="nasa-march2023">{{cite web |last1=Furfaro |first1=Emily |title=NASA's DART Data Validates Kinetic Impact as Planetary Defense Method |url=https://www.nasa.gov/feature/nasa-s-dart-data-validates-kinetic-impact-as-planetary-defense-method |website=NASA |access-date=9 March 2023 |date=28 February 2023}} {{PD-notice}}</ref><ref>{{cite journal |last1=Li |first1=Jian-Yang |last2=Hirabayashi |first2=Masatoshi |last3=Farnham |first3=Tony L. |last4=Sunshine |first4=Jessica M. |last5=Knight |first5=Matthew M. |last6=Tancredi |first6=Gonzalo |last7=Moreno |first7=Fernando |last8=Murphy |first8=Brian |last9=Opitom |first9=Cyrielle |last10=Chesley |first10=Steve |last11=Scheeres |first11=Daniel J. |last12=Thomas |first12=Cristina A. |last13=Fahnestock |first13=Eugene G. |last14=Cheng |first14=Andrew F. |last15=Dressel |first15=Linda |last16=Ernst |first16=Carolyn M. |last17=Ferrari |first17=Fabio |last18=Fitzsimmons |first18=Alan |last19=Ieva |first19=Simone |last20=Ivanovski |first20=Stavro L. |last21=Kareta |first21=Teddy |last22=Kolokolova |first22=Ludmilla |last23=Lister |first23=Tim |last24=Raducan |first24=Sabina D. |last25=Rivkin |first25=Andrew S. |last26=Rossi |first26=Alessandro |last27=Soldini |first27=Stefania |last28=Stickle |first28=Angela M. |last29=Vick |first29=Alison |last30=Vincent |first30=Jean-Baptiste |last31=Weaver |first31=Harold A. |last32=Bagnulo |first32=Stefano |last33=Bannister |first33=Michele T. |last34=Cambioni |first34=Saverio |last35=Bagatin |first35=Adriano Campo |last36=Chabot |first36=Nancy L. |last37=Cremonese |first37=Gabriele |last38=Daly |first38=R. Terik |last39=Dotto |first39=Elisabetta |last40=Glenar |first40=David A. |last41=Granvik |first41=Mikael |last42=Hasselmann |first42=Pedro H. |last43=Herreros |first43=Isabel |last44=Jacobson |first44=Seth |last45=Jutzi |first45=Martin |last46=Kohout |first46=Tomas |last47=La Forgia |first47=Fiorangela |last48=Lazzarin |first48=Monica |last49=Lin |first49=Zhong-Yi |last50=Lolachi |first50=Ramin |last51=Lucchetti |first51=Alice |last52=Makadia |first52=Rahil |last53=Epifani |first53=Elena Mazzotta |last54=Michel |first54=Patrick |last55=Migliorini |first55=Alessandra |last56=Moskovitz |first56=Nicholas A. |last57=Ormö |first57=Jens |last58=Pajola |first58=Maurizio |last59=Sánchez |first59=Paul |last60=Schwartz |first60=Stephen R. |last61=Snodgrass |first61=Colin |last62=Steckloff |first62=Jordan |last63=Stubbs |first63=Timothy J. |last64=Trigo-Rodríguez |first64=Josep M. |title=Ejecta from the DART-produced active asteroid Dimorphos |journal=Nature |date=1 March 2023 |volume=616 |issue=7957 |pages=452–456 |doi=10.1038/s41586-023-05811-4 |pmid=36858074 |pmc=10115637 |arxiv=2303.01700 |bibcode=2023Natur.616..452L |s2cid=257282549 |language=en |issn=1476-4687 |display-authors=3}}</ref> Observations show that Dimorphos lost approximately 1 million kilograms after the collision.<ref>{{cite journal |last1=Witze |first1=Alexandra |title=Asteroid lost 1 million kilograms after collision with DART spacecraft |journal=Nature |date=1 March 2023 |volume=615 |issue=7951 |pages=195 |doi=10.1038/d41586-023-00601-4 |pmid=36859675 |bibcode=2023Natur.615..195W |s2cid=257282080 |url=https://www.nature.com/articles/d41586-023-00601-4 |access-date=9 March 2023 |language=en}}</ref> Impact produced a dust plume that temporarily brightened the Didymos system and developed a {{convert|10000|km|mi|adj=on|sp=us}}-long [[Astronomy:Comet tail|dust tail]] that persisted for several months.<ref name="NOIRLab-20221003">{{cite web
 |title = SOAR Telescope Catches Dimorphos's Expanding Comet-like Tail After DART Impact
 |url = https://noirlab.edu/public/news/noirlab2223/
 |first = Charles |last= Blue
 |publisher = NOIRLab
 |date = 3 October 2022
 |accessdate = 4 February 2023}}</ref><ref name="NASA-20221215">{{cite web
 |title = Early Results from NASA's DART Mission
 |url = https://www.nasa.gov/feature/early-results-from-nasa-s-dart-mission
 |first = Jessica |last= Merzdorf
 |publisher = NASA
 |date = 15 December 2022
 |accessdate = 4 February 2023}}</ref><ref name="Li2023">{{cite journal
  |display-authors = etal
  |first1 = Jian-Yang
  |last1 = Li
  |first2 = Masatoshi
  |last2 = Hirabayashi
  |first3 = Tony
  |last3 = Farnham
  |first4 = Matthew
  |last4 = Knight
  |first5 = Gonzalo
  |last5 = Tancredi
  |first6 = Fernando
  |last6 = Moreno
  |title = Ejecta from the DART-produced active asteroid Dimorphos
  |url = https://assets.researchsquare.com/files/rs-2292349/v1/34562254-db7b-4289-a7ff-238159687528.pdf?c=1669139626
  |journal = Nature
  |date = March 2022
  |volume = 616
  |issue = 7957
  |pages = 452–456
  |doi = 10.1038/s41586-023-05811-4
  |pmid = 36858074
  |pmc = 10115637
  |arxiv = 2303.01700
  |bibcode = 2023Natur.616..452L
  |s2cid = 257282549
  |access-date = 11 March 2023
  |archive-date = 7 March 2023
  |archive-url = https://web.archive.org/web/20230307161053/https://assets.researchsquare.com/files/rs-2292349/v1/34562254-db7b-4289-a7ff-238159687528.pdf?c=1669139626
  |url-status = dead
  }}</ref>

== Observation and exploration ==
Until the age of space travel, objects in the asteroid belt could only be observed with large telescopes, their shapes and terrain remaining a mystery. The best modern ground-based telescopes and the Earth-orbiting [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]] can only resolve a small amount of detail on the surfaces of the largest asteroids. Limited information about the shapes and compositions of asteroids can be inferred from their [[Astronomy:Light curve|light curve]]s (variation in brightness during rotation) and their spectral properties. Sizes can be estimated by timing the lengths of star occultations (when an asteroid passes directly in front of a star). [[Engineering:Radar|Radar]] imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. Spacecraft flybys can provide much more data than any ground or space-based observations; sample-return missions gives insights about regolith composition.

=== Ground-based observations ===
[[File:Goldstone DSN antenna.jpg|thumb|The 70m antenna at Goldstone Observatory]]
[[File:2014 SR339 Arecibo.jpg|thumb|Radar observations of near-Earth asteroid [[Astronomy:(505657) 2014 SR339|{{mp|(505657) 2014 SR|339}}]] as seen by Arecibo]]

As asteroids are rather small and faint objects, the data that can be obtained from ground-based observations (GBO) are limited. By means of ground-based optical telescopes the visual magnitude can be obtained; when converted into the absolute magnitude it gives a rough estimate of the asteroid's size. Light-curve measurements can also be made by GBO; when collected over a long period of time it allows an estimate of the rotational period, the pole orientation (sometimes), and a rough estimate of the asteroid's shape. Spectral data (both visible-light and near-infrared spectroscopy) gives information about the object's composition, used to classify the observed asteroids. Such observations are limited as they provide information about only the thin layer on the surface (up to several micrometers).<ref name=michel>{{cite journal |last1=Michel |first1=Patrick |title=Formation and Physical Properties of Asteroids |journal=Elements |date=1 February 2014 |volume=10 |issue=1 |pages=19–24 |doi=10.2113/gselements.10.1.19 |bibcode=2014Eleme..10...19M |url=https://www.lpi.usra.edu/exploration/education/hsResearch/asteroid_101/Formation_Physical%20Properties_Asteroids.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.lpi.usra.edu/exploration/education/hsResearch/asteroid_101/Formation_Physical%20Properties_Asteroids.pdf |archive-date=2022-10-09 |url-status=live |access-date=5 May 2022}}</ref> As planetologist [[Biography:Patrick Michel|Patrick Michel]] writes:

<blockquote>Mid- to thermal-infrared observations, along with polarimetry measurements, are probably the only data that give some indication of actual physical properties. Measuring the heat flux of an asteroid at a single wavelength gives an estimate of the dimensions of the object; these measurements have lower uncertainty than measurements of the reflected sunlight in the visible-light spectral region. If the two measurements can be combined, both the effective diameter and the geometric albedo—the latter being a measure of the brightness at zero phase angle, that is, when illumination comes from directly behind the observer—can be derived. In addition, thermal measurements at two or more wavelengths, plus the brightness in the visible-light region, give information on the thermal properties. The thermal inertia, which is a measure of how fast a material heats up or cools off, of most observed asteroids is lower than the bare-rock reference value but greater than that of the lunar regolith; this observation indicates the presence of an insulating layer of granular material on their surface. Moreover, there seems to be a trend, perhaps related to the gravitational environment, that smaller objects (with lower gravity) have a small regolith layer consisting of coarse grains, while larger objects have a thicker regolith layer consisting of fine grains. However, the detailed properties of this regolith layer are poorly known from remote observations. Moreover, the relation between thermal inertia and surface roughness is not straightforward, so one needs to interpret the thermal inertia with caution.<ref name=michel/>{{Long quote|date=December 2023}}</blockquote>

Near-Earth asteroids that come into close vicinity of the planet can be studied in more details with [[Engineering:Radar|radar]]; it provides information about the surface of the asteroid (for example can show the presence of craters and boulders). Such observations were conducted by the [[Astronomy:Arecibo Observatory|Arecibo Observatory]] in Puerto Rico (305 meter dish) and Goldstone Observatory in California (70 meter dish). Radar observations can also be used for accurate determination of the orbital and rotational dynamics of observed objects.<ref name=michel/>

=== Space-based observations ===
[[File:WISE_artist_concept_(PIA17254,_crop).jpg|thumb|[[Astronomy:Wide-field Infrared Survey Explorer|WISE]] infrared space telescope]]
[[File:Asteroid 6481 Tenzing captured by JWST.gif|thumb|Asteroid 6481 Tenzing, center, is seen moving against a background of stars in this series of images taken by the [[Astronomy:James Webb Space Telescope|James Webb Space Telescope]]'s instrument NIRCam.]]
Both space and ground-based observatories conducted asteroid search programs; the space-based searches are expected to detect more objects because there is no atmosphere to interfere and because they can observe larger portions of the sky. [[Astronomy:NEOWISE|NEOWISE]] observed more than 100,000 asteroids of the main belt, [[Astronomy:Spitzer Space Telescope|Spitzer Space Telescope]] observed more than 700 near-Earth asteroids. These observations determined rough sizes of the majority of observed objects, but provided limited detail about surface properties (such as regolith depth and composition, angle of repose, cohesion, and porosity).<ref name=michel/>

Asteroids were also studied by the [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]], such as tracking the colliding asteroids in the main belt,<ref>{{cite web |title=Suspected Asteroid Collision Leaves Odd X-Pattern of Trailing Debris |url=https://hubblesite.org/contents/news-releases/2010/news-2010-07.html |website=HubbleSite.org |access-date=5 May 2022 |language=en}}</ref><ref>{{cite web |last1=Garner |first1=Rob |title=Discoveries {{!}} Highlights – Tracking Evolution in the Asteroid Belt |url=https://www.nasa.gov/content/discoveries-highlights-tracking-evolution-in-the-asteroid-belt |website=NASA |access-date=5 May 2022 |date=7 February 2017}}</ref> break-up of an asteroid,<ref>{{cite web |title=Hubble Witnesses an Asteroid Mysteriously Disintegrating |url=https://hubblesite.org/contents/news-releases/2014/news-2014-15.html |website=HubbleSite.org |access-date=5 May 2022 |language=en}}</ref> observing an [[Astronomy:Active asteroid|active asteroid]] with six comet-like tails,<ref>{{cite web |title=NASA's Hubble Sees Asteroid Spout Six Comet-like Tails |url=https://hubblesite.org/contents/news-releases/2013/news-2013-52.html |website=HubbleSite.org |access-date=5 May 2022 |language=en}}</ref> and observing asteroids that were chosen as targets of dedicated missions.<ref>{{cite web |title=Hubble Images of Asteroids Help Astronomers Prepare for Spacecraft Visit |url=https://hubblesite.org/contents/news-releases/2007/news-2007-27.html |website=HubbleSite.org |access-date=5 May 2022 |language=en}}</ref><ref>{{cite web |title=Hubble Reveals Huge Crater on the Surface of the Asteroid Vesta |url=https://hubblesite.org/contents/news-releases/1997/news-1997-27.html |website=HubbleSite.org |access-date=5 May 2022 |language=en}}</ref>

=== Space probe missions ===

According to [[Biography:Patrick Michel|Patrick Michel]]

<blockquote>The internal structure of asteroids is inferred only from indirect evidence: bulk densities measured by spacecraft, the orbits of natural satellites in the case of asteroid binaries, and the drift of an asteroid's orbit due to the Yarkovsky thermal effect. A spacecraft near an asteroid is perturbed enough by the asteroid's gravity to allow an estimate of the asteroid's mass. The volume is then estimated using a model of the asteroid's shape. Mass and volume allow the derivation of the bulk density, whose uncertainty is usually dominated by the errors made on the volume estimate. The internal porosity of asteroids can be inferred by comparing their bulk density with that of their assumed meteorite analogues, dark asteroids seem to be more porous (>40%) than bright ones. The nature of this porosity is unclear.<ref name="michel" /></blockquote>

==== Dedicated missions ====
The first asteroid to be photographed in close-up was [[Astronomy:951 Gaspra|951 Gaspra]] in 1991, followed in 1993 by [[Astronomy:243 Ida|243 Ida]] and its moon Dactyl, all of which were imaged by the [[Engineering:Galileo (spacecraft)|''Galileo'' probe]] en route to [[Astronomy:Jupiter|Jupiter]]. Other asteroids briefly visited by spacecraft en route to other destinations include [[Astronomy:9969 Braille|9969 Braille]] (by ''[[Astronomy:Deep Space 1|Deep Space 1]]'' in 1999), [[Astronomy:5535 Annefrank|5535 Annefrank]] (by ''[[Engineering:Stardust (spacecraft)|Stardust]]'' in 2002), [[Astronomy:2867 Šteins|2867 Šteins]] and [[Astronomy:21 Lutetia|21 Lutetia]] (by the [[Engineering:Rosetta (spacecraft)|''Rosetta'' probe]] in 2008), and [[Astronomy:4179 Toutatis|4179 Toutatis]] (China's lunar orbiter ''[[Astronomy:Chang'e 2|Chang'e 2]]'', which flew within {{cvt|2|mi|km|order=flip}} in 2012).

The first dedicated asteroid probe was NASA's ''[[Astronomy:NEAR Shoemaker|NEAR Shoemaker]]'', which photographed [[Astronomy:253 Mathilde|253 Mathilde]] in 1997, before entering into orbit around [[Astronomy:433 Eros|433 Eros]], finally landing on its surface in 2001. It was the first spacecraft to successfully orbit and land on an asteroid.<ref name=twofirsts>{{cite news|url=https://solarsystem.nasa.gov/missions/near-shoemaker/in-depth/|title=NEAR Shoemaker|publisher=NASA|accessdate=April 26, 2021}}</ref> From September to November&nbsp;2005, the Japanese ''Hayabusa'' probe studied [[Astronomy:25143 Itokawa|25143 Itokawa]] in detail and returned samples of its surface to Earth on 13&nbsp;June 2010, the first asteroid sample-return mission. In 2007, [[Organization:NASA|NASA]] launched the [[Engineering:Dawn (spacecraft)|''Dawn'']] spacecraft, which orbited [[Astronomy:4 Vesta|4 Vesta]] for a year, and observed the dwarf planet [[Astronomy:Ceres (dwarf planet)|Ceres]] for three years.

''[[Astronomy:Hayabusa2|Hayabusa2]]'', a probe launched by [[Organization:JAXA|JAXA]] 2014, orbited its target asteroid  [[Astronomy:162173 Ryugu|162173 Ryugu]] for more than a year and took samples that were delivered to Earth in 2020. The spacecraft is now on an extended mission and expected to arrive at a new target in 2031.

NASA launched the [[Astronomy:OSIRIS-REx|OSIRIS-REx]] in 2016, a sample return mission to asteroid [[Astronomy:101955 Bennu|101955 Bennu]]. In 2021, the probe departed the asteroid with a sample from its surface. Sample was delivered to Earth in September 2023. The spacecraft continues its extended mission, designated OSIRIS-APEX, to explore near-Earth asteroid Apophis in 2029.

In 2021, NASA launched [[Astronomy:Double Asteroid Redirection Test|Double Asteroid Redirection Test]] (DART), a mission to test technology for defending Earth against potential hazardous objects. DART deliberately crashed into the [[Astronomy:Minor-planet moon|minor-planet moon]] [[Astronomy:Dimorphos|Dimorphos]] of the double asteroid [[Astronomy:65803 Didymos|Didymos]] in September 2022 to assess the potential of a spacecraft impact to deflect an asteroid from a collision course with Earth.<ref>{{Cite web|last=Potter|first=Sean|date=2021-11-23|title=NASA, SpaceX Launch DART: First Test Mission to Defend Planet Earth|url=http://www.nasa.gov/press-release/nasa-spacex-launch-dart-first-test-mission-to-defend-planet-earth|access-date=2021-12-04|website=NASA}}</ref> In October, NASA declared DART a success, confirming it had shortened Dimorphos' orbital period around Didymos by about 32 minutes.<ref>{{Cite web |last=Bardan |first=Roxana |date=2022-10-11 |title=NASA Confirms DART Mission Impact Changed Asteroid's Motion in Space |url=http://www.nasa.gov/press-release/nasa-confirms-dart-mission-impact-changed-asteroid-s-motion-in-space |access-date=2022-10-11 |website=NASA}}</ref>

<gallery mode="packed" heights="150" caption="Asteroid-dedicated space probes">
File:Hayabusa2 Ion thruster.jpg|''Hayabusa2''
File:Dawn - PIA12033.jpg|''Dawn''
File:Lucy-PatroclusMenoetius-art.png|''Lucy''
File:PSYCHE.jpg|''Psyche''
</gallery>

==== Planned missions ====
[[File:Small Asteroids and Comets Visited by Spacecraft.jpg|thumb|upright=1.3|Asteroids and comets visited by spacecraft as of 2019 (except Ceres and Vesta), to scale]]

* NASA's ''[[Engineering:Lucy (spacecraft)|Lucy]]'', launched in 2021, would visit eight asteroids, one from the [[Astronomy:Asteroid belt|main belt]] and seven [[Astronomy:Jupiter trojan|Jupiter trojan]]s; it is the first mission to trojans. The main mission would start in 2027.<ref>{{citation-attribution|1={{cite web |last=Hille|first=Karl|date=2019-10-21|title=NASA's Lucy Mission Clears Critical Milestone|url=http://www.nasa.gov/feature/goddard/2019/lucy-mission-clears-critical-milestone|publisher=NASA|access-date=2020-12-05}} }}</ref><ref>{{citation-attribution|1={{cite web|title=Lucy: The First Mission to the Trojan Asteroids|date=21 April 2017|url=https://www.nasa.gov/mission_pages/lucy/overview/index|publisher=NASA|access-date=2021-10-16|archive-date=6 December 2020|archive-url=https://web.archive.org/web/20201206213030/https://www.nasa.gov/mission_pages/lucy/overview/index|url-status=dead}} }}</ref>
* NASA's ''[[Engineering:Psyche (spacecraft)|Psyche]]'', launched in October 2023, will study the large metallic asteroid [[Astronomy:16 Psyche|of the same name]], and will arrive there in 2029.
* ESA's ''[[Astronomy:Hera (space mission)|Hera]]'', planned for launch in 2024, will study the results of the DART impact. It will measure the size and morphology of the crater, and momentum transmitted by the impact, to determine the efficiency of the deflection produced by DART.
* JAXA's [[Astronomy:DESTINY+|DESTINY+]] is a mission for a flyby of the [[Astronomy:Geminids|Geminids]] meteor shower parent body [[Astronomy:3200 Phaethon|3200 Phaethon]], as well as various minor bodies. Its launch is planned for 2024.<ref name="dlr-20201112">{{cite web|url=https://www.dlr.de/content/en/articles/news/2020/04/20201112_destiny-germany-and-japan-begin-new-asteroid-mission.html|title=DESTINY+ – Germany and Japan begin new asteroid mission|publisher=German Aerospace Center (DLR)|date=12 November 2020|access-date=15 November 2020}}</ref>
* CNSA's ''[[Astronomy:Tianwen-2|Tianwen-2]]'' is planned to launch in 2025.<ref>{{Cite web |last=Andrew Jones published |date=2022-05-18 |title=China to launch Tianwen 2 asteroid-sampling mission in 2025 |url=https://www.space.com/china-tianwen2-asteroid-sampling-mission-2025-launch |access-date=2022-09-29 |website=Space.com |language=en}}</ref> It will use [[Engineering:Solar electric propulsion|solar electric propulsion]] to explore the co-orbital near-Earth asteroid [[Astronomy:469219 Kamoʻoalewa|469219 Kamoʻoalewa]] and the [[Astronomy:Active asteroid|active asteroid]] 311P/PanSTARRS. The spacecraft will collect samples of the regolith of Kamo'oalewa.<ref name="nature20190430">{{cite journal |last=Gibney |first=Elizabeth |url=https://www.nature.com/articles/d41586-019-01390-5 |title=China plans mission to Earth's pet asteroid |journal=Nature |date=30 April 2019 |access-date=4 June 2019 |doi=10.1038/d41586-019-01390-5|pmid=32346150 |s2cid=155198626 }}</ref>

== Asteroid mining ==
{{Main|Astronomy:Asteroid mining|Astronomy:Colonization of the asteroids}}
[[File:Artist Concept - Astronaut Performs Tethering Maneuvers at Asteroid.jpg|thumb|Artist's concept of a crewed mission to an asteroid]]

The concept of asteroid mining was proposed in 1970s. Matt Anderson defines successful asteroid mining as "the development of a mining program that is both financially self-sustaining and profitable to its investors".<ref>{{cite journal |last1=Anderson |first1=Matt |title=Mining Near Earth Asteroids |journal=Planetary Sciences Class |date=1 May 2015 |url=http://www.chara.gsu.edu/~thenry/PLANETS/paper.anderson.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.chara.gsu.edu/~thenry/PLANETS/paper.anderson.pdf |archive-date=2022-10-09 |url-status=live |access-date=13 April 2022}}</ref> It has been suggested that asteroids might be used as a source of materials that may be rare or exhausted on Earth,<ref>{{cite journal |last1=Anderson |first1=Scot W |last2=Christensen |first2=Korey |last3=LaManna |first3=Julia |title=The development of natural resources in outer space |journal=Journal of Energy & Natural Resources Law |date=3 April 2019 |volume=37 |issue=2 |pages=227–258 |doi=10.1080/02646811.2018.1507343 |bibcode=2019JENRL..37..227A |s2cid=169322274 |url=https://www.hoganlovells.com/~/media/hogan-lovells/pdf/2018/the_development_of_natural_resouces_in_outer_space_august_2018.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.hoganlovells.com/~/media/hogan-lovells/pdf/2018/the_development_of_natural_resouces_in_outer_space_august_2018.pdf |archive-date=2022-10-09 |url-status=live |access-date=13 April 2022}}</ref> or materials for constructing [[Astronomy:Space habitat|space habitat]]s. Materials that are heavy and expensive to launch from Earth may someday be mined from asteroids and used for space manufacturing and construction.<ref>{{cite web |title=How Asteroid Mining Will Work |url=https://science.howstuffworks.com/asteroid-mining.htm |website=HowStuffWorks |access-date=13 April 2022 |language=en |date=10 November 2000}}</ref><ref>{{cite web |last1=Wall |first1=Mike |title=Asteroid-Mining Project Aims for Deep-Space Colonies |url=https://www.space.com/19368-asteroid-mining-deep-space-industries.html |website=Space.com |access-date=13 April 2022 |language=en |date=22 January 2013}}</ref>

As resource depletion on Earth becomes more real, the idea of extracting valuable elements from asteroids and returning these to [[Astronomy:Earth|Earth]] for profit, or using space-based resources to build [[Physics:Space-based solar power|solar-power satellites]] and space habitats,<ref>{{cite web | url=http://settlement.arc.nasa.gov/spaceres/IV-2.html | title=Retrieval of Asteroidal Materials | publisher=NASA | website=Space Resources and Space Settlements, 1977 Summer Study at NASA Ames Research Center, Moffett Field, California | year=1979 | author1=Brian O'Leary | author2=Michael J. Gaffey | author3=David J. Ross | author4=Robert Salkeld | name-list-style=amp | access-date=2011-09-29 | archive-date=2019-05-24 | archive-url=https://web.archive.org/web/20190524014201/https://settlement.arc.nasa.gov/spaceres/IV-2.html | url-status=dead }}</ref><ref>{{cite web | url=http://ssi.org/reading/papers/space-studies-institute-roadmap/ | title=A Space Roadmap: Mine the Sky, Defend the Earth, Settle the Universe | publisher=[[Organization:Space Studies Institute|Space Studies Institute]] | year=2002 | access-date=September 19, 2011 | author=Lee Valentine | archive-date=August 7, 2019 | archive-url=https://web.archive.org/web/20190807051056/http://ssi.org/reading/papers/space-studies-institute-roadmap/ | url-status=live }}</ref> becomes more attractive. Hypothetically, water processed from ice could refuel orbiting [[Chemistry:Propellant depot|propellant depot]]s.<ref>{{cite journal | title=A captured asteroid : Our David's stone for shielding earth and providing the cheapest extraterrestrial material | year=2006 |author1=Didier Massonnet |author2=Benoît Meyssignac | doi=10.1016/j.actaastro.2006.02.030 | volume=59 | issue=1–5 | journal=Acta Astronautica | pages=77–83|bibcode = 2006AcAau..59...77M }}</ref><ref name="Kiss">{{cite web |url=http://kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf |title=Asteroid Retrieval Feasibility Study |publisher=Keck Institute for Space Studies, California Institute of Technology, Jet Propulsion Laboratory |date=12 April 2012 |author1=John Brophy |author2=Fred Culick |author3=Louis Friedman |display-authors=0 |access-date=19 April 2012 |archive-date=31 May 2017 |archive-url=https://web.archive.org/web/20170531053431/http://www.kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf |url-status=live }}</ref>

From the [[Astronomy:Astrobiology|astrobiological]] perspective, asteroid prospecting could provide scientific data for the search for extraterrestrial intelligence (SETI). Some astrophysicists have suggested that if advanced extraterrestrial civilizations employed asteroid mining long ago, the hallmarks of these activities might be detectable.<ref>{{Cite web|archive-url=https://web.archive.org/web/20110408062400/http://smithsonianscience.org/2011/04/evidence-of-asteroid-mining-in-our-galaxy-may-lead-to-the-discovery-of-extraterrestrial-civilizations/|url=http://smithsonianscience.org/2011/04/evidence-of-asteroid-mining-in-our-galaxy-may-lead-to-the-discovery-of-extraterrestrial-civilizations/|url-status=live|title=Evidence of asteroid mining in our galaxy may lead to the discovery of extraterrestrial civilizations|website=Smithsonian Science|date=2011-04-05|archive-date=2011-04-08|publisher=Smithsonian Institution}}</ref><ref>{{Cite web|url=https://www.centauri-dreams.org/2011/03/29/asteroid-mining-a-marker-for-seti/|title=Asteroid Mining: A Marker for SETI?|last=Gilster|first=Paul|date=2011-03-29|website=www.centauri-dreams.org|access-date=2019-12-26|archive-date=2019-12-26|archive-url=https://web.archive.org/web/20191226113900/https://www.centauri-dreams.org/2011/03/29/asteroid-mining-a-marker-for-seti/|url-status=live}}</ref><ref>{{Cite journal |arxiv = 1103.5369|last1 = Marchis|first1 = Franck|title = Extrasolar Asteroid Mining as Forensic Evidence for Extraterrestrial Intelligence|journal = International Journal of Astrobiology|volume = 10|issue = 4|pages = 307–313|last2 = Hestroffer|first2 = Daniel|last3 = Descamps|first3 = Pascal|last4 = Berthier|first4 = Jerome|last5 = Bouchez|first5 = Antonin H|last6 = Campbell|first6 = Randall D|last7 = Chin|first7 = Jason C. Y|last8 = van Dam|first8 = Marcos A|last9 = Hartman|first9 = Scott K|last10 = Johansson|first10 = Erik M|last11 = Lafon|first11 = Robert E|author12 = David Le Mignant|author13 = Imke de Pater|last14 = Stomski|first14 = Paul J|last15 = Summers|first15 = Doug M|last16 = Vachier|first16 = Frederic|last17 = Wizinovich|first17 = Peter L|last18 = Wong|first18 = Michael H|year = 2011|doi = 10.1017/S1473550411000127|bibcode = 2011IJAsB..10..307F|s2cid = 119111392}}</ref>

== Threats to Earth ==

[[File:SmallAsteroidImpacts-Frequency-Bolide-20141114.jpg|thumb|Frequency of [[Astronomy:Bolide|bolide]]s, small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere]]

There is increasing interest in identifying asteroids whose orbits cross [[Astronomy:Earth|Earth]]'s, and that could, given enough time, collide with Earth. The three most important groups of [[Astronomy:Near-Earth asteroid|near-Earth asteroid]]s are the [[Astronomy:Apollo asteroid|Apollos]], [[Astronomy:Amor asteroid|Amors]], and [[Astronomy:Aten asteroid|Atens]].

The [[Astronomy:Near-Earth object|near-Earth]] asteroid [[Astronomy:433 Eros|433 Eros]] had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: [[Astronomy:1221 Amor|1221 Amor]], [[Astronomy:1862 Apollo|1862 Apollo]], [[Astronomy:2101 Adonis|2101 Adonis]], and finally [[Astronomy:69230 Hermes|69230 Hermes]], which approached within 0.005&nbsp;[[Astronomy:Astronomical unit|AU]] of [[Astronomy:Earth|Earth]] in 1937. Astronomers began to realize the possibilities of Earth impact.

Two events in later decades increased the alarm: the increasing acceptance of the [[Biology:Alvarez hypothesis|Alvarez hypothesis]] that an [[Astronomy:Impact event|impact event]] resulted in the [[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction]], and the 1994 observation of Comet Shoemaker-Levy 9 crashing into [[Astronomy:Impact events on Jupiter|Jupiter]]. The U.S. military also declassified the information that its [[Engineering:Military satellite|military satellite]]s, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to ten meters across.

All of these considerations helped spur the launch of highly efficient surveys, consisting of charge-coupled device ([[Engineering:Charge-coupled device|CCD]]) cameras and computers directly connected to telescopes. {{As of|2011}}, it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered.<ref name=nasa_neo/> {{as of|2018|10|29}}, the LINEAR system alone had discovered 147,132 asteroids.<ref>{{cite web |title=Minor Planet Discover Sites |publisher=International Astronomical Union |department=Minor Planet Center |url=https://minorplanetcenter.net//iau/lists/MPDiscSites.html |access-date=27 December 2018}}</ref> Among the surveys, 19,266&nbsp;near-Earth asteroids have been discovered<ref>{{cite web |title=Unusual Minor Planets |publisher=International Astronomical Union |department=Minor Planet Center |url=https://minorplanetcenter.net//iau/lists/Unusual.html |access-date=27 December 2018}}<!--- using the "close approach" quote ---></ref> including almost 900&nbsp;more than {{cvt|1|km|1}} in diameter.<ref>{{cite web |series=Discovery Statistics |title=Cumulative Totals |date=20 December 2018 |publisher=NASA |department=Jet Propulsion Laboratory |url=https://cneos.jpl.nasa.gov/stats/totals.html |access-date=27 December 2018}}</ref>

In June 2018, the National Science and Technology Council warned that the United States is unprepared for an asteroid impact event, and has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare.<ref name="GIZ-20180621" /><ref name="ICARUS-220180522" /><ref name="NYT-20180614">{{cite news
 |last=Chang |first=Kenneth
 |date=14 June 2018
 |title=Asteroids and adversaries: Challenging what NASA knows about space rocks
 |newspaper={{wipe|The New York Times}}
 |url=https://www.nytimes.com/2018/06/14/science/asteroids-nasa-nathan-myhrvold.html
 |access-date=22 June 2018
}}</ref> According to expert testimony in the United States Congress in 2013, [[Organization:NASA|NASA]] would require at least five years of preparation before a mission to intercept an asteroid could be launched.<ref name="US-Congress-20130410">{{cite report |collaboration=House Committee on Science, Space, and Technology, One Hundred Thirteenth Congress, First Session |date=19 March 2013 |title=Threats from Space: A review of U.S. Government efforts to track and mitigate asteroids and meteors |volume=Part&nbsp;I and Part&nbsp;II |page=147 |series=Hearing before the Committee on Science, Space, and Technology |publisher=House of Representatives |url=http://www.gpo.gov/fdsys/pkg/CHRG-113hhrg80552/pdf/CHRG-113hhrg80552.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.gpo.gov/fdsys/pkg/CHRG-113hhrg80552/pdf/CHRG-113hhrg80552.pdf |archive-date=2022-10-09 |url-status=live |access-date=26 November 2018}}</ref>

=== Asteroid deflection strategies ===
[[File:Dart-poster3.jpg|thumb|Double Asteroid Redirection Test in 2022 demonstrated that spacecraft impact is a viable option for planetary defense.]]
Various collision avoidance techniques have different trade-offs with respect to metrics such as overall performance, cost, failure risks, operations, and technology readiness.<ref>{{cite journal|last1=Canavan|first1=G. H |last2=Solem|first2=J. C.|year=1992|title=Interception of near-Earth objects|journal=Mercury|issn=0047-6773|volume=21|issue=3|pages=107–109|url=https://www.researchgate.net/publication/253052410|bibcode=1992Mercu..21..107C}}</ref> There are various methods for changing the course of an asteroid/comet.<ref name="HallRoss">C. D. Hall and [[Biography:I. Michael Ross|I. M. Ross]], "Dynamics and Control Problems in the Deflection of Near-Earth Objects", ''Advances in the Astronautical Sciences, Astrodynamics 1997'', Vol. 97, Part I, 1997, pp. 613–631.</ref> These can be differentiated by various types of attributes such as the type of mitigation (deflection or fragmentation), energy source (kinetic, electromagnetic, gravitational, solar/thermal, or nuclear), and approach strategy ({{Anchor|interception2016-01-26}}interception,<ref>{{cite journal|last=Solem|first=J. C.|year=1993|title=Interception of comets and asteroids on collision course with Earth|journal=Journal of Spacecraft and Rockets|volume=30|issue=2|pages=222–228|doi=10.2514/3.11531|bibcode=1993JSpRo..30..222S|url=https://digital.library.unt.edu/ark:/67531/metadc1090076/}}</ref><ref>Solem, J. C.; Snell, C. (1994). "[https://books.google.com/books?id=xXWZolI9NkUC&dq=Terminal+intercept+for+less+than+one+orbital+snell&pg=PA1013 Terminal intercept for less than one orbital period warning] {{webarchive |url=https://web.archive.org/web/20160506210107/https://books.google.com/books?id=xXWZolI9NkUC&pg=PA1013&lpg=PA1013&dq=Terminal+intercept+for+less+than+one+orbital+snell#v=onepage&q=Terminal%20intercept%20for%20less%20than%20one%20orbital%20snell&f=false |date=May 6, 2016 }}", a chapter in ''Hazards Due to Comets and Asteroids'', Geherels, T., ed. (University of Arizona Press, Tucson), pp. 1013–1034.</ref> rendezvous, or remote station).

Strategies fall into two basic sets: fragmentation and delay.<ref name="HallRoss"/><ref>{{cite journal|last=Solem|first=J. C.|year=2000|title=Deflection and disruption of asteroids on collision course with Earth|journal=Journal of the British Interplanetary Society |volume=53|pages=180–196|url=http://www.jbis.org.uk/paper.php?p=2000.53.180 |bibcode=2000JBIS...53..180S}}</ref> Fragmentation concentrates on rendering the impactor harmless by fragmenting it and scattering the fragments so that they miss the Earth or are small enough to burn up in the atmosphere. Delay exploits the fact that both the Earth and the impactor are in orbit. An impact occurs when both reach the same point in space at the same time, or more correctly when some point on Earth's surface intersects the impactor's orbit when the impactor arrives. Since the [[Astronomy:Earth|Earth]] is approximately 12,750&nbsp;km in diameter and moves at approx. 30&nbsp;km per second in its orbit, it travels a distance of one planetary diameter in about 425 seconds, or slightly over seven minutes. Delaying, or advancing the impactor's arrival by times of this magnitude can, depending on the exact geometry of the impact, cause it to miss the Earth.<ref name="RossParkPorter">{{cite journal|last1=Ross|first1=I. M.|last2=Park|first2=S.-Y.|last3=Porter|first3=S. E.|title=Gravitational Effects of Earth in Optimizing Delta-V for Deflecting Earth-Crossing Asteroids|journal=Journal of Spacecraft and Rockets|volume=38|issue=5|date=2001|pages=759–764|hdl=10945/30321|url=https://calhoun.nps.edu/bitstream/handle/10945/30321/AIAA-3743-490.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://calhoun.nps.edu/bitstream/handle/10945/30321/AIAA-3743-490.pdf |archive-date=2022-10-09 |url-status=live|access-date=2019-08-30|citeseerx=10.1.1.462.7487|doi=10.2514/2.3743|s2cid=123431410 }}</ref>

"[[Astronomy:1566 Icarus#Project Icarus|Project Icarus]]" was one of the first projects designed in 1967 as a contingency plan in case of collision with [[Astronomy:1566 Icarus|1566 Icarus]]. The plan relied on the new [[Engineering:Saturn V|Saturn V]] rocket, which did not make its first flight until after the report had been completed. Six Saturn V rockets would be used, each launched at variable intervals from months to hours away from impact. Each rocket was to be fitted with a single 100-megaton nuclear warhead as well as a modified Apollo Service Module and uncrewed Apollo Command Module for guidance to the target. The warheads would be detonated 30 meters from the surface, deflecting or partially destroying the asteroid. Depending on the subsequent impacts on the course or the destruction of the asteroid, later missions would be modified or cancelled as needed. The "last-ditch" launch of the sixth rocket would be 18 hours prior to impact.<ref name="Portree">{{cite magazine |author=David S. F. Portree |title=MIT Saves the World: Project Icarus (1967) |url=https://www.wired.com/2012/03/mit-saves-the-world-project-icarus-1967/ |magazine=Wired |access-date=21 October 2013}}</ref>

== Fiction ==

Asteroids and the asteroid belt are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places human beings might colonize, resources for extracting minerals, hazards encountered by spacecraft traveling between two other points, and as a threat to life on Earth or other inhabited planets, dwarf planets, and natural satellites by potential impact.

== See also ==
* [[Astronomy:Exoasteroid|Exoasteroid]]
* [[Astronomy:List of asteroid close approaches to Earth|List of asteroid close approaches to Earth]]
* [[Astronomy:List of exceptional asteroids|List of exceptional asteroids]]
* [[Astronomy:Lost minor planet|Lost minor planet]]
* [[Astronomy:Meanings of minor-planet names|Meanings of minor-planet names]]

== Notes ==
{{notelist|1}}

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}}

== Further reading ==
* {{cite book |editor1-last=Badescu |editor1-first=Viorel |title=Asteroids : prospective energy and material resources |date=2013 |publisher=Springer |location=Berlin |doi=10.1007/978-3-642-39244-3 |bibcode=2013aste.book.....B |isbn=978-3-642-39244-3 |url=https://link.springer.com/book/10.1007/978-3-642-39244-3 |language=en}}
* {{cite book |last1=Barnes-Svarney |first1=Patricia L. |title=Asteroid : Earth destroyer or New Frontier? |date=2003 |publisher=Basic Books |location=Cambridge, Mass. |isbn=978-0-7382-0885-5}}
* {{cite book |editor1-last=Binzel |editor1-first=Richard P. |editor1-link=Richard P. Binzel |editor2-last=Gehrels |editor2-first=Tom |editor2-link=Tom Gehrels |editor3-last=Matthews |editor3-first=Mildred Shapley |editor3-link=Mildred Shapley Matthew|title=Asteroids II |date=1989 |publisher=University of Arizona Press |location=Tucson |isbn=978-0-8165-1123-5}}
* {{cite book |editor1-last=Bottke |editor1-first=William F. |editor1-link=William F. Bottke |editor2-last=Cellino |editor2-first=Alberto |editor3-last=Paolicchi |editor3-first=Paolo |editor4-last=Binzel |editor4-first=Richard P. |editor4-link=Richard P. Binzel |title=Asteroids III |date=2002 |publisher=University of Arizona Press |location=Tucson |isbn=978-0-8165-4651-0 |url=https://books.google.com/books?id=JwHTyO6IHh8C }}
* {{cite book |last1=Kowal |first1=Charles T. |title=Asteroids : their nature and utilization |date=1996 |publisher=J. Wiley |location=Chichester, England |isbn=978-0-471-96039-3 |edition=2nd}}
* {{cite journal |last1=Metzger |first1=Philip T. |last2=Sykes |first2=Mark V. |last3=Stern |first3=Alan |last4=Runyon |first4=Kirby |title=The Reclassification of Asteroids from Planets to Non-Planets |journal=Icarus |date=February 2019 |volume=319 |pages=21–32 |doi=10.1016/j.icarus.2018.08.026 |arxiv=1805.04115 |bibcode=2019Icar..319...21M |s2cid=119206487 }}
* {{cite book |editor1-last=Michel |editor1-first=Patrick |editor1-link=Patrick Michel |editor2-last=DeMeo |editor2-first=Francesca E. |editor3-last=Bottke |editor3-first=William F. |editor3-link=William F. Bottke |title=Asteroids IV |date=2015 |publisher=Lunar and Planetary Institute |location=Houston |isbn=978-0-8165-3218-6}}
* {{cite book |last1=Peebles |first1=Curtis |title=Asteroids: A History |date=2000 |publisher=Smithsonian Institution Press |location=Washington,DC |isbn=978-1-56098-389-7}}

== External links ==
* {{cite web |title=Alphabetical list of minor planet names |url=http://www.minorplanetcenter.org/iau/lists/MPNames.html |publisher=International Astronomical Union |department=Minor Planet Center}}
* {{cite web |title=Asteroid articles in Planetary Science Research Discoveries |url=http://www.psrd.hawaii.edu/Archive/Archive-Asteroids.html |department=Planetary Science |publisher=University of Hawaii}}
* {{cite web |title=JPL Asteroid Watch site |publisher=[[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] |url=http://www.jpl.nasa.gov/asteroidwatch/}}
* [http://www.nasa.gov/asteroid-and-comet-watch NASA Asteroid and Comet Watch site]

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